Solar cells incorporating light harvesting arrays

ABSTRACT

A solar cell incorporates a light harvesting array that comprises: (a) a first substrate comprising a first electrode; and (b) a layer of light harvesting rods electrically coupled to the first electrode, each of the light harvesting rods comprising a polymer of Formula I: 
     
       
         X 1 X m+1 ) m   (I) 
       
     
     wherein m is at least 1, and may be from two, three or four to 20 or more; X 1  is a charge separation group (and preferably a porphyrinic macrocycle, which may be one ligand of a double-decker sandwich compound) having an excited-state of energy equal to or lower than that of X 2 ; and X 2  through X m+1  are chromophores (and again are preferably porphyrinic macrocycles).

This invention was made with Government support under Grant No.DE-FG02-96ER14632 from the Department of Energy and Grant No. GM36238from the National Institutes of Health. The Government has certainrights to this invention.

FIELD OF THE INVENTION

The present invention concerns solar cells, particularly regenerativesolar cells, and light harvesting arrays useful in such solar cells.

BACKGROUND OF THE INVENTION

Molecular approaches for converting sunlight to electrical energy have arich history with measurable “photoeffects” reported as early as 1887 inVienna (Moser, J. Montash. Chem. 1887, 8, 373.). The most promisingdesigns were explored in considerable detail in the 1970's (Gerischer,H. Photochem. Photobiol. 1972, 16, 243; Gerischer, H. Pure Appl. Chem.1980, 52, 2649; Gerischer, H.; Willig, F. Top. Curr. Chem. 1976, 61,31). Two common approaches are shown in FIG. 1, both of whichincorporate molecules that selectively absorb sunlight, termedphotosensitizers or simply sensitizers (S), covalently bound toconductive electrodes. Light absorption by the sensitizer creates anexcited state, S*, that injects an electron into the electrode and thenoxidizes a species in solution. The right hand side depicts a simplifiedphotoelectrosynthetic cell. This cell produces both electrical power andchemical products. Many of the molecular approaches over the past fewdecades were designed to operate in the manner shown with the goal ofsplitting water into hydrogen and oxygen. Shown on the left hand side isa regenerative cell that converts light into electricity with no netchemistry. In the regenerative solar cell shown, the oxidation reactionsthat take place at the photoanode are reversed at the dark cathode.

The principal difficulty with these solar cell designs is that amonolayer of a molecular sensitizer on a flat surface does not absorb asignificant fraction of incident visible light. As a consequence, evenif the quantum yields of electron transfer are high on an absorbedphoton basis, the solar conversion efficiency will be impractically lowbecause so little light is absorbed. Early researchers recognized thisproblem and tried to circumvent it by utilizing thick films ofsensitizers. This strategy of employing thick absorbing layers wasunsuccessful as intermolecular excited-state quenching in the thicksensitizer film decreased the yield of electron injection into theelectrode.

One class of thick film sensitizers is provided by the so-called organicsolar cells (Tang, C. W. and Albrecht, A. C. J. Chem. Phys. 1975, 63,953-961). Here a 0.01 to 5 μm thick film, typically comprised ofphthalocyanines, perylenes, chlorophylls, porphyrins, or mixturesthereof, is deposited onto an electrode surface and is employed in wetsolar cells like those shown, or as solid-state devices where a secondmetal is deposited on top of the organic film. The organic layer isconsidered to be a small bandgap semiconductor with either n- or p-typephotoconductivity and the proposed light-to-electrical energy conversionmechanisms incorporate excitonic energy transfer among the pigments inthe film toward the electrode surface where interfacial electrontransfer takes place. However, the importance of these proposedmechanistic steps is not clear. Increased efficiencies that result fromvectorial energy transfer among the pigments have not been convincinglydemonstrated. Furthermore, the reported excitonic diffusion lengths areshort relative to the penetration depth of the light. Accordingly, mostof the light is absorbed in a region where the energy cannot betranslated to the semiconductor surface. The excitons are also readilyquenched by impurities or incorporated solvent, leading to significantchallenges in reproducibility and fabrication. The state-of-the-artorganic solar cells are multilayer organic “heterojunction” films ordoped organic layers that yield ˜2% efficiencies under low irradiance,but the efficiency drops markedly as the irradiance approaches that ofone sun (Forrest, S. R. et al., J. Appl. Phys. 1989, 183, 307; Schon, J.H. et al., Nature 2000, 403, 408).

Another class of molecular-based solar cells are the so-calledphotogalvanic cells that were the hallmark molecular level solar energyconversion devices of the 1940's -1950's (Albery, W. J. Acc. Chem. Res.1982, 15, 142). These cells are distinguished from those discussed abovein that the excited sensitizer does not undergo interfacial electrontransfer. The cells often contain sensitizers embedded in a membranethat allows ion transfer and charge transfer; the membrane physicallyseparates two dark metal electrodes and photogenerated redoxequivalents. The geometric arrangement precludes direct excited-stateelectron transfer from a chromophore to or from the electrodes. Rather,intermolecular charge separation occurs and the reducing and oxidizingequivalents diffuse to electrodes where thermal interfacial electrontransfer takes place. A transmembrane Nernst potential can be generatedby photodriven electron transfer occurring in the membrane. Inphotoelectrosynthetic galvanic cells, chemical fuels may be formed aswell. This general strategy for dye sensitization of electrodes has beenemployed in many guises over the years, but the absolute efficienciesremain very low. Albery concluded that an efficiency of ˜13%theoretically could be achieved in an aqueous regenerative photogalvaniccell. However, efficiencies realized to date are typically less than 2%.

In 1991, a breakthrough was reported by Gratzel and O'Regan (O'Regan, B.et al., J. Phys. Chem. 1990, 94, 8720; O'Regan, B. and Grätzel, M.Nature 1991, 353, 737). By replacing the planar electrodes with a thickporous colloidal semiconductor film, the surface area for sensitizerbinding increased by over 1000-fold. Gratzel and O'Regan demonstratedthat a monolayer of sensitizer coating the semiconductor particlesresulted in absorption of essentially all of the incident light, andincident photon-to-electron energy conversion efficiencies were unity atindividual wavelengths of light in regenerative solar cells.Furthermore, a global efficiency of ˜5% was realized under air-mass 1.5illumination conditions; this efficiency has risen to a confirmed 10.69%today (Gratzel, M. in “Future Generation Photovoltaic Technologies”McConnell, R. D.; AIP Conference Proceedings 404, 1997, page 119). These“Gratzel” solar cells have already found niche markets and arecommercially available in Europe.

These high surface area colloidal semiconductor films (Gratzel cells)achieve a high level of absorption but also have the followingsignificant drawbacks. (1) A liquid junction is required for highefficiency (because the highly irregular surface structure makesdeposition of a solid-state conductive layer essentially impossible).(2) The colloidal semiconductor films require high temperature annealingsteps to reduce internal resistances. Such high temperatures imposesevere limitations on the types of conductive substrates that can beused. For example, polymeric substrates that melt below the requiredannealing temperatures cannot be used. (3) Significant losses areassociated with transporting charge through the thick semiconductorfilms. These losses do not appreciably decrease the photocurrent, buthave a large effect on the voltage output and thus the power isdecreased significantly (Hagfeldt, A.; Grätzel, M. Chem. Rev. 1995, 95,49). Accordingly, there remains a need for new molecular approaches tothe construction of solar cells.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides, among other things, a lightharvesting array useful for the manufacture of solar cells. The lightharvesting array comprises:

(a) a first substrate comprising a first electrode; and

(b) a layer of light harvesting rods electrically coupled to the firstelectrode, each of the light harvesting rods comprising a polymer ofFormula I:

X¹X^(m+1))_(m)  (I)

wherein:

m is at least 1, and may be from two, three or four to 20 or more;

X¹ is a charge separation group having an excited-state of energy equalto or lower than that of X²; and

X² through X^(m+1) are chromophores.

In light harvesting rods of Formula I herein, X¹ preferably comprises aporphyrinic macrocycle, which may be in the form of a double-deckersandwich compound. Further, X² through X^(m+1) also preferably compriseporphyrinic macrocycles.

In one preferred embodiment of the light harvesting rods of Formula Iherein, at least one of (e.g., two, three, a plurality of, the majorityof or all of) X¹ through X^(m+1) is/are selected from the groupconsisting of chlorins, bacteriochlorins, and isobacteriochlorins.

A particular embodiment of a light harvesting array as described aboveprovides for the movement of holes in the opposite direction ofexcited-state energy along some or all of the length of the lightharvesting rods, and comprises:

(a) a first substrate comprising a first electrode; and

(b) a layer of light harvesting rods electrically coupled to the firstelectrode, each of the light harvesting rods comprising a polymer ofFormula I:

X¹X^(m+1))_(m)  (I)

 wherein:

m is at least 1 (typically two, three or four to twenty or more);

X¹ is a charge separation group having an excited-state of energy equalto or lower than that of X²,

X²through X^(m+1) are chromophores; and

X¹ through X^(m+1) are selected so that, upon injection of either anelectron or hole from X¹ into the first electrode, the correspondinghole or electron from X¹ is transferred to at least X², and optionallyto X³, X⁴, and all the way through X^(m+1). In a currently preferredembodiment, X¹ through X^(m+1) are selected so that, upon injection ofan electron from X¹ into the first electrode, the corresponding holefrom X¹ is transferred to at least X², and optionally through X^(m+1).

Light-harvesting arrays provide intense absorption of light and deliverthe resulting excited state to a designated location within themolecular array. There are a variety of applications of light-harvestingarrays. Light-harvesting arrays can be used as components of low-levellight detection systems, especially where control is desired over thewavelength of light that is collected. Light-harvesting arrays can beused as input elements in optoelectronic devices, and as an input unitand energy relay system in molecular-based signaling systems. Oneapplication of the latter includes use in molecular-based fluorescencesensors. The molecular-based sensor employs a set of probe groups (whichbind an analyte) attached to a molecular backbone that undergoesexcited-state energy transfer. The binding of a single analyte to anyone of the probe groups yields a complex that can quench the excitedstate that freely migrates along the backbone (i.e., exciton). Thequenching phenomenon results in diminished fluorescence from themolecular backbone. Because only one bound analyte can cause thequenching phenomenon, the sensitivity is much higher than if there was a1:1 ratio of probe groups and fluorescence groups. Previously, suchmolecular-based fluorescence sensors have employed UV or near-UVabsorbing chromophores in the molecular backbone. The light-harvestingarrays described herein are ideally suited as components for a new classof molecular-based fluorescence sensors that absorb (and fluoresce)strongly in the visible and near-infrared region.

A particular application of the light-harvesting arrays described hereinis in solar cells. A solar cell as described herein typically comprises:

(a) a first substrate comprising a first electrode;

(b) a second substrate comprising a second electrode, with the first andsecond substrate being positioned to form a space therebetween, and withat least one of (i) the first substrate and the first electrode and (ii)the second substrate and the second electrode being transparent;

(c) a layer of light harvesting rods electrically coupled to the firstelectrode, each of the light harvesting rods comprising a polymer ofFormula I:

X¹X^(m+1))_(m)  (I)

 wherein:

m is at least 1 (and typically two, three or four to twenty or more);

X¹ is a charge separation group having an excited-state of energy equalto or lower than that of X²;

X² through X^(m+1) are chromophores; and

X¹ is electrically coupled to the first electrode; the solar cellfurther comprising

(d) an electrolyte in the space between the first and second substrates.A mobile charge carrier can optionally be included in the electrolyte.

In a particular embodiment of the foregoing (sometimes referred to as“design II” herein), the solar cell comprises:

(a) a first substrate comprising a first electrode;

(b) a second substrate comprising a second electrode, with the first andsecond substrate being positioned to form a space therebetween, and withat least one of (i) the first substrate and the first electrode and (ii)the second substrate and the second electrode being transparent;

(c) a layer of light harvesting rods electrically coupled to the firstelectrode, each of the light harvesting rods comprising a polymer ofFormula I:

X¹X^(m+1))_(m)  (I)

 wherein:

m is at least 1 (and typically two, three or four to twenty or more);

X¹ is a charge separation group having an excited-state of energy equalto or lower than that of X²;

X² through X^(m+1) are chromophores;

X¹ is electrically coupled to the first electrode; and

X¹ through X^(m+1) are selected so that, upon injection of either anelectron or hole from X¹ into the first electrode, the correspondinghole or electron from X¹ is transferred to X² (and optionally to X³, X⁴,and in some cases all the way to X^(m+1)); the solar cell furthercomprising

(d) an electrolyte in the space between the first and second substrates;and

(e) optionally, but preferably, a mobile charge carrier in theelectrolyte. In a currently preferred embodiment, X¹ through X^(m+1) areselected so that, upon injection of an electron from X¹ into the firstelectrode, the corresponding hole from X¹ is transferred to X² throughX^(m+1).

Another particular embodiment (sometimes referred to as “design III”herein) of a solar cell as described above comprises:

(a) a first substrate comprising a first electrode;

(b) a second substrate comprising a second electrode, with the first andsecond substrate being positioned to form a space therebetween, and withat least one of (i) the first substrate and the first electrode and (ii)the second substrate and the second electrode being transparent;

(c) a layer of light harvesting rods electrically coupled to the firstelectrode, each of the light harvesting rods comprising a polymer ofFormula I:

X¹X^(m+1))_(m)  (I)

 wherein:

m is at least 1 (and typically two, three or four to twenty or more);

X¹ is a charge separation group having an excited-state of energy equalto or lower than that of X²;

X² through X^(m+1) are chromophores;

X¹ is electrically coupled to the first electrode; and

X^(m+1) is electrically coupled to the second electrode; the solar cellfurther comprising

(d) an electrolyte in the space between the first and second substrates.Again, X¹ through X^(m+1) may be selected so that, upon injection of anelectron or hole (preferably an electron) from X¹ into the firstelectrode, the corresponding hole or electron from X¹ is transferred toX², or optionally to X³ or X⁴ or all the way through X^(m+1).

A variety of different electrical devices comprised of a solar cell asdescribed above having circuits (typically resistive loads) electricallycoupled thereto can be produced with the solar cells of the invention,as discussed in greater detail below.

The present invention is explained in greater detail in the drawingsherein and the specification set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Diagrams of the two common molecular approaches for light toelectrical energy conversion.

FIG. 2. General diagram of linear chromophore arrays (light harvestingrods).

FIG. 3. Energy migration along the light harvesting rod and use of amobile charge carrier to regenerate the charge separation unit followingelectron injection (Design I).

FIG. 4. Energy migration and hole hopping in opposite directions (DesignII).

FIG. 5. Energy migration and hole hopping in opposite directions withthe light harvesting rod sandwiched between the two electrodes (DesignIII).

FIG. 6. Double-decker sandwich molecules that may serve as sensitizers.

FIG. 7. Sensitization mechanisms for an n-type semiconductor by asensitizer S. Here E_(CB) and E_(VB) are the semiconductor conductionband and valence band edge, respectively. E_(f) is the Fermi-level ofthe semiconductor. E^(o)(S^(+/0)) and E^(o)(S^(+/*)) are the formalreduction potentials of the ground and excited state, respectively.Gerischer's distributions of sensitizer donor and acceptor levels arealso shown.

FIG. 8. Simplified representation of the sensitization mechanism of TiO₂by a sensitizer S. Light excitation of the sensitizer forms an excitedstate S*, which injects an electron injection into the semiconductorwith rate constant, k_(inj). The oxidized sensitizer, S⁺, is thenregenerated by an external electron donor (e.g., iodide), with rateconstant k_(red). The V_(oc) is the open-circuit photovoltage, whichrepresents the maximum Gibbs free energy that can be abstracted from thecell under conditions of constant illumination. Competing with powerproduction is charge recombination, k_(cr), that can occur (from thesemiconductor) to the oxidized sensitizer or the oxidized product of themobile charge carrier (e.g., triiodide).

FIG. 9. A regenerative solar cell designed to function like thatdescribed for FIG. 8, except that a solid-state hole conductor replacesthe iodide/triiodide redox-active electrolyte.

FIG. 10. Examples of building blocks that can be assembled intochromophore arrays.

FIG. 11. Synthetic approach for preparing linear chromophore arrays.

FIG. 12. Rational synthesis of a porphyrin dimeric building block forpreparing chromophore arrays.

FIG. 13. Solid-phase synthesis using Suzuki coupling to preparep-phenylene linked porphyrin containing arrays.

FIG. 14. Solid-phase synthesis using Suzuki coupling to preparep-phenylene linked chlorin containing arrays.

FIG. 15. Bifunctional building blocks for use in Suzuki polymerizations.

FIG. 16. Rational synthesis of a bifunctional porphyrin building blockfor use in Suzuki polymerizations.

FIG. 17. Solid-phase synthesis of meso,meso-linked porphyrin containingarrays with an attached carboxy handle.

FIG. 18. Solid-phase synthesis of meso,meso-linked porphyrin containingarrays with an attached ethyne handle.

FIG. 19. Attachment of a meso,meso-linked array to a zirconium doubledecker sandwich molecule.

FIG. 20. Example of energy migration but no hole migration in achromophore array.

FIG. 21. Example of energy migration and hole migration in oppositedirections in a chromophore array.

FIG. 22. Example of a cascade of energy migration and hole migration inopposite directions in a chromophore array. Hole migration occurs over adefined region of the array.

FIG. 23. Another example of solid-phase synthesis using Suzuki couplingto prepare p-phenylene linked chlorin containing arrays with an ethynehandle.

FIG. 24. Attachment of a p-phenylene linked chlorin containing array toa zirconium double decker sandwich molecule.

FIG. 25. Example of reversible energy migration and irreversible holemigration in a chromophore array.

FIG. 26. A diphenylethyne-linked bacteriochlorin containing array.

FIG. 27. Chlorin building blocks that have substituents (functionalhandles) at two of the meso positions, and none at the β positions.

FIG. 28. In a porphyrin having an a_(2u) HOMO (which has electrondensity predominantly at the meso positions and little at the βpositions), faster rates (2.5-10-fold) are observed with linkers at themeso rather than β positions.

FIG. 29. Four different chlorin building blocks, and chlorinnomenclature showing IUPAC-IUB ring labels A-D.

FIG. 30. Orientation of the transition dipole moment of thelong-wavelength absorption band in free base chlorin and metallochlorin.

FIG. 31. Pairwise interaction of chlorin building blocks uponincorporation in covalently linked arrays.

FIG. 32. The highest occupied molecular orbital of a chlorin is an a₂orbital, which places electron density at each of the meso andnon-reduced β sites.

FIG. 33 illustrates the synthesis of a trans-chlorin building block withtwo β substituents.

FIG. 34A. The synthesis of the new β-substituted Eastern half forchlorin synthesis.

FIG. 34B. The synthesis of the new β-substituted Eastern half forchlorin synthesis, extending the route shown in FIG. 34A.

FIG. 35 illustrates the synthesis of the new β-substituted Western halffor a chlorin building block.

FIG. 36. Other chlorin building blocks that are accessible via this samesynthetic strategy shown above, and that have substantially the samephysical properties.

FIG. 37. The synthesis of the trans meso-substituted chlorin buildingblocks (type III) by an extension of the route for preparing chlorinsbearing adjacent (cis) meso-substituted chlorins.

FIG. 38. A second route to trans meso-substituted chlorin buildingblocks (type III).

FIG. 39. Various meso-substituted chlorin building blocks that can beaccessed in the synthetic manner described above.

FIG. 40. The relationship of antenna complexes and reaction center forthe production of holes and electrons from excitation energy flowingfrom the antenna.

FIG. 41. Light-harvesting arrays that absorb light and undergo efficientintramolecular energy transfer.

FIG. 42. Here a novel means of moving the oxidizing equivalent away fromthe charge-separation unit is designed. Energy flows along thelight-harvesting array to the charge-separation unit, while theoxidizing equivalent (hole) flows in the reverse direction from the CSUto a site in the antenna where subsequent electron-transfer reactionscan take place.

FIG. 43. The design of FIG. 42 has two significant ramifications. (1)Only two access channels are required at the CSU: one for emanation ofthe electrons, and one where excitation energy flows in and oxidizingequivalents (holes) flow out.

FIG. 44 illustrates a linear array of zinc porphyrins bearing differentmeso substituents.

FIG. 45 illustrates a linear array of Mg and Zn porphyrins bearingdifferent meso substituents.

FIG. 46 illustrates a linear array of metallochlorins bearing differentmeso substituents.

FIG. 47 illustrates a linear array of porphyrins and chlorins bearingdifferent meso substituents.

FIG. 48 illustrates a linear array of β-substituted chlorins andmeso-substituted chlorins.

FIG. 49 illustrates a linear array of porphyrin, chlorin, andphthalocyanine components.

FIG. 50 illustrates a cataract linear array employing domains comprisedof multiple isoenergetic pigments.

FIG. 51 illustrates reactions suitable for preparing light-harvestingrod oligomers.

FIG. 52. In situ polymerization yielding a light-harvesting rod on asurface (e.g., Au or TiO₂) that will serve as one of the solar cellelectrodes.

FIG. 53 illustrates the synthesis of meso-substituted chlorins bypreviously disclosed techniques.

FIG. 54 illustrates the synthesis of β-substituted chlorin eastern half(EH) precursors.

FIG. 55 further illustates the synthesis of β-substituted chlorineastern half precursors.

FIG. 56 illustrates the synthesis of a β-substituted chlorin westernhalf (WH).

FIG. 57 illustrates the synthesis of a β-substituted chlorin.

FIG. 58 illustrates the synthesis of a trans β-substituted chlorin.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The solar cells described herein entail the use of linear chromophorearrays (light harvesting rods) that provide strong absorption of light.In addition, and when desired, the solar cells described herein providefor energy migration and charge migration in opposite directions. Thusthe chromophore arrays absorb light and may exhibit an intrinsicmolecular level rectification in the flow of excited-state energy andground-state holes.

Without wishing to be limiting of the invention, it is noted that somepotential advantages of the solar cells described herein include thefollowing: thin (e.g., rods not greater than 500 or even 200 nanometersin length), lightweight, portable, flexible, good efficiency,solid-state (in one embodiment), ease of fabrication, and rationalmolecular design. Indeed, it is contemplated that the inventiondescribed herein will permit, where desired, quantitative conversion ofincident photons to electrons at individual wavelengths of light andglobal efficiencies >5% under solar illumination.

I. Definitions

The following terms and phrases are used herein: A substrate as usedherein is preferably a solid material (which may be flexible or rigid)suitable for the attachment of one or more molecules. Substrates can beformed of materials including, but not limited to glass, organicpolymers, plastic, silicon, minerals (e.g. quartz), semiconductingmaterials, ceramics, metals, etc. The substrate may be in any suitableshape, including flat, planar, curved, rod-shaped, etc. The substratemay be inherently conductive and serve itself as an electrode, or anelectrode may be formed on or connected to the substrate by any suitablemeans (e.g., deposition of a gold layer or a conductive oxide layer).Either or both of the substrates in the solar cells may be transparent(that is, wavelengths of light that excite the chromophores can passthrough the substrate and corresponding electrode, even if they arevisually opaque). In light-harvesting arrays, the substrate andelectrode may be of any suitable type. One of the substrates may beopaque with respect to the wavelengths of light that excite thechromophores. One of the substrates may be reflective or provided with areflective coating so that light that passes through the arrays or rodsis reflected back to the arrays or rods.

The term “electrode” refers to any medium capable of transporting charge(e.g. electrons) to and/or from a light harvesting rod. Preferredelectrodes are metals (e.g., gold, aluminum), non-metals (e.g.,conductive oxides, carbides, sulfide, selinides, tellurides, phosphides,and arsenides such as cadmium sulfide, cadmium telluride, tungstendiselinide, gallium arsenide, gallium phosphide, etc.), and conductiveorganic molecules. The electrodes can be manufactured to virtually any2-dimensional or 3-dimensional shape.

The term “conductive oxide” as used herein refers to any suitableconductive oxide including binary metal oxides such as tin oxide, indiumoxide, titanium oxide, copper oxide, and zinc oxide, or ternary metaloxides such as strontium titanate and barium titanate. Other examples ofsuitable conductive oxides include but are not limited to indium tinoxide, titanium dioxide, tin oxide, gallium indium oxide, zinc oxide,and zinc indium oxide. The metal oxide semiconductors may be intrinsicor doped, with trace amounts of materials, to control conductivity.

The term “heterocyclic ligand” as used herein generally refers to anyheterocyclic molecule consisting of carbon atoms containing at leastone, and preferably a plurality of, hetero atoms (e.g., N, O, S, Se,Te), which hetero atoms may be the same or different, and which moleculeis capable of forming a sandwich coordination compound with anotherheterocyclic ligand (which may be the same or different) and a metal.Such heterocyclic ligands are typically macrocycles, particularlytetrapyrrole derivatives such as the phthalocyanines, porphyrins, andporphyrazines.

The term “porphyrinic macrocycle” refers to a porphyrin or porphyrinderivative. Such derivatives include porphyrins with extra ringsortho-fused, or ortho-perifused, to the porphyrin nucleus, porphyrinshaving a replacement of one or more carbon atoms of the porphyrin ringby an atom of another element (skeletal replacement), derivatives havinga replacement of a nitrogen atom of the porphyrin ring by an atom ofanother element (skeletal replacement of nitrogen), derivatives havingsubstituents other than hydrogen located at the peripheral (meso-, β-)or core atoms of the porphyrin, derivatives with saturation of one ormore bonds of the porphyrin (hydroporphyrins, e.g, chlorins,bacteriochlorins, isobacteriochlorins, decahydroporphyrins, corphins,pyrrocorphins, etc.), derivatives obtained by coordination of one ormore metals to one or more porphyrin atoms (metalloporphyrins),derivatives having one or more atoms, including pyrrolic andpyrromethenyl units, inserted in the porphyrin ring (expandedporphyrins), derivatives having one or more groups removed from theporphyrin ring (contracted porphyrins, e.g., corrin, corrole) andcombinations of the foregoing derivatives (e.g. phthalocyanines,porphyrazines, naphthalocyanines, subphthalocyanines, and porphyrinisomers). Preferred porphyrinic macrocycles comprise at least one5-membered ring.

The term porphyrin refers to a cyclic structure typically composed offour pyrrole rings together with four nitrogen atoms and two replaceablehydrogens for which various metal atoms can readily be substituted. Atypical porphyrin is hemin.

A “chlorin” is essentially the same as a porphyrin, but differs from aporphyrin in having one partially saturated pyrrole ring. The basicchromophore of chlorophyll, the green pigment of plant photosynthesis,is a chlorin.

A “bacteriochlorin” is essentially the same as a porphyrin, but differsfrom a porphyrin in having two partially saturated non-adjacent (i.e.,trans) pyrrole rings.

An “isobacteriochlorin” is essentially the same as a porphyrin, butdiffers from a porphyrin in having two partially saturated adjacent(i.e., cis) pyrrole rings.

The terms “sandwich coordination compound” or “sandwich coordinationcomplex” refer to a compound of the formula L^(n)M^(n−1), where each Lis a heterocyclic ligand such as a porphyrinic macrocycle, each M is ametal, n is 2 or more, most preferably 2 or 3, and each metal ispositioned between a pair of ligands and bonded to one or more heteroatom (and typically a plurality of hetero atoms, e.g, 2, 3, 4, 5) ineach ligand (depending upon the oxidation state of the metal). Thussandwich coordination compounds are not organometallic compounds such asferrocene, in which the metal is bonded to carbon atoms. The ligands inthe sandwich coordination compound are generally arranged in a stackedorientation (i.e., are generally cofacially oriented and axially alignedwith one another, although they may or may not be rotated about thataxis with respect to one another). See, e.g., D. Ng and J. Jiang, Chem.Soc. Rev. 26, 433-442 (1997). Sandwich coordination compounds may be“homoleptic” (wherein all of the ligands L are the same) or“heteroleptic” (wherein at least one ligand L is different from theother ligands therein).

The term “double-decker sandwich coordination compound” refers to asandwich coordination compound as described above where n is 2, thushaving the formula L¹-M¹-L², wherein each of L¹ and L² may be the sameor different. See, e.g., J. Jiang et al., J. Porphyrins Phthalocyanines3, 322-328 (1999).

The term “multiporphyrin array” refers to a discrete number of two ormore covalently-linked porphyrinic macrocycles. The multiporphyrinarrays can be linear, cyclic, or branched, but are preferably linearherein. Light harvesting rods herein are preferably multiporphyrinarrays. The light harvesting rods or multiporphyrin arrays may be linear(that is, all porphyrinic macrocycles may be linked in trans) or maycontain one or more bends or “kinks” (for example, by including one ormore non-linear linkers in a light-harvesting rod, or by including oneor more cis-substituted porphyrinic macrocycles in the light harvestingrod) Some of the porphyrinic macrocycles may further include additionalligands, particularly porphyrinic macrocycles, to form sandwichcoordination compounds as described further below. The rods optionallybut preferably are oriented substantially perpendicularly to either, andmost preferably both, of the first and second electrodes.

“Chromophore” means a light-absorbing unit which can be a unit within amolecule or can comprise the entire molecule. Typically a chromophore isa conjugated system (alternating double and single bonds which caninclude non-bonded electrons but is not restricted to alternating doubleand single bonds since triple and single bonds, since mixtures ofalternating triple/double and single bonds also constitute chromophores.A double or triple bond alone constitutes a chromophore. Heteroatoms canbe included in a chromophore.). Examples of chromophores include thecyclic 18 pi-electron conjugated system that imparts color toporphyrinic pigments, the linear system of alternating double and singlebonds in the visual pigment retinal, or the carbonyl group in acetone.

“Charge separation group” and “charge separation unit” refer tomolecular entities that upon excitation (by direct absorption or energytransfer from another absorber) displace an electron to another part ofthe same molecule, or transfer an electron to a different molecule,semiconductor, or metal. The “charge separation group” and “chargeseparation unit” results in storage of some fraction of the excitedstate energy upon displacement or transfer of an electron. Typically the“charge separation group” and “charge separation unit” is located at theterminus of a light-harvesting array or rod, from which excited-stateenergy is received. The “charge separation group” and “charge separationunit” facilitates or causes conversion of the excited-state energy intoa separate electron and hole or an electron-hole pair. The electron canbe injected into the semiconductor by the “charge separation group” or“charge separation unit”. It is feasible that the “charge separationgroup” and “charge separation unit” could extract an electron from adifferent molecule or semiconductor, thereby creating a negative chargeon the “charge separation group” and “charge separation unit” and a holein the other molecule or semiconductor. The reaction center of bacterialphotosynthesis is a premier example of a “charge separation group” or“charge separation unit”. Synthetic porphyrin-quinone orporphyrin-buckyball molecules also function to absorb light and utilizethe resulting energy to separate charge.

The term “substituent” as used in the formulas herein, particularlydesignated by S or S^(n) where n is an integer, in a preferredembodiment refer to electron-rich or electron-deficient groups(subunits) that can be used to adjust the redox potential(s) of thesubject compound. Preferred substituents include, but are not limitedto, H, aryl, phenyl, cycloalkyl, alkyl, halogen, alkoxy, alkylthio,perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro,amino, alkylamino, acyl, sulfoxyl, sulfonyl, amido, and carbamoyl. Inpreferred embodiments, a substituted aryl group is attached to aporphyrin or a porphyrinic macrocycle, and the substituents on the arylgroup are selected from the group consisting of aryl, phenyl,cycloalkyl, alkyl, halogen, alkoxy, alkylthio, perfluoroalkyl,perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino,acyl, sulfoxyl, sulfonyl, amido, and carbamoyl. Additional substituentsinclude, but are not limited to, 4-chlorophenyl,4-trifluoromethylphenyl, and 4-methoxyphenyl. Preferred substituentsprovide a redox potential range of less than about 5 volts, preferablyless than about 2 volts, more preferably less than about 1 volt.

The term “aryl” refers to a compound whose molecules have the ringstructure characteristic of benzene, naphthalene, phenanthrene,anthracene, etc. (i.e., either the 6-carbon ring of benzene or thecondensed 6-carbon rings of the other aromatic derivatives). Forexample, an aryl group may be phenyl (C₆H₅) or naphthyl (C₁₀H₇). It isrecognized that the aryl group, while acting as substituent can itselfhave additional substituents (e.g. the substituents provided for S^(n)in the various formulas herein).

The term “alkyl” refers to a paraffinic hydrocarbon group which may bederived from an alkane by dropping one hydrogen from the formula.Examples are methyl (CH₃—), ethyl (C₂H₅—), propyl (CH₃CH₂CH₂—),isopropyl ((CH₃)₂CH—).

The term “halogen” refers to one of the electronegative elements ofgroup VIIA of the periodic table (fluorine, chlorine, bromine, iodine,astatine).

The term “perfluoroalkyl” refers to an alkyl group where every hydrogenatom is replaced with a fluorine atom.

The term “perfluoroaryl” refers to an aryl group where every hydrogenatom is replaced with a fluorine atom.

The term “pyridyl” refers to an aryl group where one CR unit is replacedwith a nitrogen atom.

The term “sulfoxyl” refers to a group of composition RS(O)— where R issome alkyl, aryl, cycloalkyl, perfluoroalkyl, or perfluoroaryl group.Examples include, but are not limited to methylsulfoxyl, phenylsulfoxyl,etc.

The term “sulfonyl” refers to a group of composition RSO₂— where R issome alkyl, aryl, cycloalkyl, perfluoroalkyl, or perfluoroaryl group.Examples include, but are not limited to methylsulfonyl, phenylsulfonyl,p-toluenesulfonyl, etc.

The term “carbamoyl” refers to the group of composition R¹(R²)NC(O)—where R¹ and R² are H or some alkyl, aryl, cycloalkyl, perfluoroalkyl,or perfluoroaryl group. Examples include, but are not limited toN-ethylcarbamoyl, N,N-dimethylcarbamoyl, etc.

The term “amido” refers to the group of composition R¹CON(R²)— where R¹and R² are H or some alkyl, aryl, cycloalkyl, perfluoroalkyl, orperfluoroaryl group. Examples include, but are not limited to acetamido,N-ethylbenzamido, etc.

The term “acyl” refers to an organic acid group in which the —OH of thecarboxyl group is replaced by some other substituent (RCO—). Examplesinclude, but are not limited to acetyl, benzoyl, etc.

In preferred embodiments, when a metal is designated by “M” or “M^(n)”,where n is an integer, it is recognized that the metal may be associatedwith a counterion.

A linker is a molecule used to couple two different molecules, twosubunits of a molecule, or a molecule to a substrate. When all arecovalently linked, they form units of a single molecule.

The term “electrically coupled” when used with reference to a lightharvesting rod and electrode, or to chromophores, charge separationgroups and electrodes, refers to an association between that group ormolecule and the coupled group or electrode such that electrons movefrom the storage medium/molecule to the electrode or from the electrodeto the molecule and thereby alter the oxidation state of the storagemolecule. Electrical coupling can include direct covalent linkagebetween the storage medium/molecule and the electrode, indirect covalentcoupling (e.g. via a linker), direct or indirect ionic bonding betweenthe storage medium/molecule and the electrode, or other bonding (e.g.hydrophobic bonding). In addition, no actual bonding may be required andthe light harvesting rod may simply be contacted with the electrodesurface. There also need not necessarily be any contact between theelectrode and the light harvesting rod where the electrode issufficiently close to the light harvesting rod to permit electrontunneling between the medium/molecule and the electrode.

“Excited-state energy” refers to the energy stored in the chromophore ina metastable state following absorption of light (or transfer of energyfrom an absorber). For an excited singlet (triplet) state, the magnitudeof the “excited-state energy” is estimated by energy of the shortestwavelength fluorescence (phosphorescence) band. The magnitude of the“excited-state energy” is greater than or equal to the energy of theseparated electron and hole following charge separation.

Electrolytes used to carry out the present invention may be aqueous ornon-aqueous electrolytes, including polymer electrolytes. Theelectrolyte may comprise or consist of a solid, in which latter case thesolar cell can be produced devoid of liquid in the space between thefirst and second substrates. The electrolyte consists of or comprises asubstance that increases the electrical conductivity of a carriermedium. Most electrolytes are salts or ionic compounds. Examples includesodium chloride (table salt), lithium iodide, or potassium bromide inwater; tetrabutylammonium hexafluorophosphate or tetraethylammoniumperchlorate in acetonitrile or dichloromethane; or an ionic polymer in agel.

“Mobile charge carriers” refers to an ion, molecule, or other speciescapable of translating charges (electrons or holes) between the twoelectrodes in a solar cell. Examples include quinones in water, moltensalts, and iodide in a polymer gel such as polyacrylonitrile. Examplesof mobile charge carriers include, but are not limited to, iodide,bromide, tetramethyl-1,4-phenylenediamine,tetraphenyl-1,4-phenylenediamine, p-benzoquinone, C₆₀, C₇₀, pentacene,tetrathiafulvalene, and methyl viologen.

II. Solar Cells Containing Light-harvesting Rods

A. Introduction.

The objective of developing an ultrathin solar cell with high absorptioncoefficient is met with the use of linear chromophore arrays that serveas light-harvesting (LH) rods. The generic design of linear chromophorearrays is shown in FIG. 2. The pigments (i.e., chromophore-containingmolecules) are joined covalently via linkers to create the lineararchitecture. At one end of the array is situated the sensitizer orcharge-separation unit (CSU). The CSU is also attached to the electrodevia a linker and functional group designated by Y. At the distal end ofthe light-harvesting rod is a capping group designated by Z. The cappinggroup can consist of a simple alkyl or aryl substituent, or can comprisea linker for attachment to a surface or counterelectrode. Uponattachment of the linear LH/CSU molecules to the electrode viaattachment group Y, the rods will orient in a more or less verticalfashion. In so doing, the linear rod-like architecture enables amultilayer stack of pigments where each pigment in a rod is held apartfrom the neighboring pigments in the same rod via linker L. The packingpatterns and distances between rods are controlled by substituentsintegral to the pigments. Generically these are referred to as“chromophore arrays”, a term used interchangeably with linearlight-harvesting rods (both terms indicate a linear architecture oflinked pigments that absorb light efficiently and funnel energy (andholes) in a controlled manner). Note that the terms sensitizer orcharge-separation are used unit interchangeably; the latter emphasizesthe fact that the photoexcited agent (sensitizer) that injects theelectron into the semiconductor can be comprised of multiple units(e.g., porphyrin-chlorin, chlorin-quinone, bacteriochlorin-buckyball).

Three distinct designs are described for the chromophore arrays (videinfra). Implicit in all the design schemes is that fact that the arrayswill harvest a large fraction of incident solar irradiation. Thestrategy of employing a monolayer of molecular sensitizers on a planarelectrode surface has historically been flawed because of the smallfraction of incident solar light that is absorbed. The describedinvention conceptualizes a new molecular approach wherein theprefabricated chromophore arrays will be organized on an electrodesurface. By assembling the arrays perpendicular to the electrodesurface, monolayer coverages will result in significantly increasedlight absorption. For example, phthalocyanines typically have extinctioncoefficients of ˜250,000 M⁻¹cm¹⁻¹ in the red part of the visible region(600-700 nm depending on metalation state). A monolayer of suchphthalocyanines on a flat surface corresponds to ˜10⁻¹⁰ mol/cm² and willabsorb about 5.6% of the incident light. An array of 20 phthalocyanineswith additive absorption (i.e., no new absorption bands due toaggregation and/or electronic interactions) spatially arranged to occupythe same surface area would absorb 68% of the incident light. If thenumber of phthalocyanines was increased to 40, or the surface roughnessfactor of the electrode was two, 90% of the incident light would beabsorbed. Many electrode surfaces are inherently rough so that amonolayer of 20-chromophore arrays (i.e., arrays each comprised of 20chromophores) would result in essentially quantitative light absorption.This projection compares very favorably with the surface roughnessfactor of ˜1000 necessary for efficient light harvesting, as iscurrently employed in the Gratzel-type cells.

In Design I, the LH/CSU rods are attached to one electrode viaattachment group Y (FIG. 3). The cell includes mobile (i.e., diffusive)charge carriers. The linear LH rods, generally comprising 5-20 pigments,absorb light. Excitation energy transfer among pigments in the rod, bythrough-space and/or through-bond mechanisms, results in energy reachingthe CSU (illustrated in step 2, FIG. 3). The excited CSU then injects anelectron into the conduction band of the electrode (step 4). Theresulting hole resides on the CSU and cannot migrate into the LH rodbecause the oxidation potential of the CSU is lower than that of theimmediately adjacent pigments in the LH rod. Diffusion of a mobilecharge carrier in close proximity of the oxidized CSU results inelectron/hole transfer, regenerating the CSU and leaving the hole on themobile charge carrier. The hole then moves by diffusion of the mobilecharge carrier and/or subsequent hole-transfer processes among mobilecharge carriers until the counterelectrode is reached at the distal endof the LH rod (near Z; not shown).

In Design II, the LH/CSU rods are attached to one electrode viaattachment group Y (FIG. 4). The cell includes mobile (i.e., diffusive)charge carriers. All features are the same as in Design I with theexception that the hole formed in the CSU (upon electron injection intothe electrode) can migrate into the linear LH rod. This has twoconsequences. (1) The lifetime of the charge-separated state isincreased giving a commensurate decrease in charge recombinationprocesses at the CSU-electrode interface. (2) The mobile charge carrierscan access the hole at sites distant from the electrode surface. Sitesdistant from the surface are anticipated to be more accessible therebyfacilitating hole transfer and migration (via diffusion) to thecounterelectrode.

In Design III, the LH/CSU rods are attached to one electrode viaattachment group Y (FIG. 5). The opposite end of each rod is attached tothe counterelectrode. No mobile (i.e., diffusive) charge carriers arepresent in the cell (though an electrolyte can be present). Absorptionof light, energy migration among pigments, and electron transfer at theCSU occur identically with those processes in Designs I and II. However,the hole in the CSU resulting from electron injection into the electrodemigrates by hole-hopping among pigments in the LH rod and then transfersto the counterelectrode. There are several ramifications to this design.(1) No diffusive charge carriers are present in the cell. (2) Only twodistinct access channels are required at the CSU; one for electrontransfer to the electrode, and one provided by the LH rod for transferin of excitation energy and transfer out of the resulting holes. Incontrast, Design I requires three access channels at the CSU, one forinward migration of energy, one for outward transfer of an electron, andone for the mobile charge carrier to gain access to the hole. Theabsence of mobile charge carriers in Design III results in a solid-statesolar cell.

In prior studies of light-harvesting phenomena, star-shaped arrayscomprised of porphyrins and phthalocyanines have been created (Li, J.;Lindsey, J. S. J. Org. Chem. 1999, 64, 9101-9108; Li, J et al., J. Org.Chem. 1999, 64, 9090-9100; Li, F. et al., J. Mater. Chem. 1997, 7,1245-1262), a cluster of boron-dipyrrin dyes surrounding a porphyrin(Li, F. et al J. Am. Chem. Soc. 1998, 120, 10001-10017), and a lineararray of four porphyrins and one boron-dipyrrin (Wagner, R. W.; Lindsey,J. S. J. Am. Chem. Soc. 1994, 116, 9759-9760), and cyclic arrays (Li, J.et al., J. Am. Chem. Soc. 1999, 121, 8927-8940). The effects ofdifferent metals in metalloporphyrins in modulating the rate of energytransfer also has been studied (Hascoat, P. et al., Inorg. Chem. 1999,38, 4849-4853). Also, the effects of different linkers on the rate ofenergy transfer (Hsiao, J.-S. et al., J. Am. Chem. Soc. 1996, 118,11181-11193; Yang, S. I. et al., J. Phys. Chem. 1998, 102, 9426-9436),orbital composition (Strachan, J. P. et al., J. Am. Chem. Soc. 1997,119, 11191-11201), and the position of location of the linker on theporphyrinic pigment (Yang, S. I. et al., J. Am. Chem. Soc. 1999, 121,4008-4018) have been characterized. Simulations of energy migration inlinear chromophore arrays also have been performed to evaluate theperformance of various molecular architectural designs (Van Patten, P.G. et al., J. Phys. Chem. B 1998, 102, 4209-4216). These syntheticlight-harvesting molecules absorb strongly in the visible region andundergo highly efficient energy transfer. As part of these studies,examination of the properties of the oxidized complexes revealed rapidhole-hopping among the components (Seth, J. et al., J. Am. Chem. Soc.1994, 116, 10578-10592; Seth, J. et al., J. Am. Chem. Soc. 1996, 118,11194-11207). The features herein proposed to elicit in the design ofthe linear chromophoric arrays described in this invention aresupported, but not at all anticipated, in this body of prior work. Thesynthetic methods for preparing light-harvesting arrays are sufficientfor someone skilled in the art to prepare the molecules describedherein. In particular, there are extensive methods for preparingporphyrin building blocks (a) Lindsey, J. S. et al., Tetrahedron 1994,50, 8941-8968. (b) Lindsey, J. S. In The Porphyrin Handbook; Kadish, K.M.; Smith, K. M.; Guilard, R., Eds.; Academic Press, San Diego, Calif.2000, Vol. 1, pp 45-118; Cho, W.-S. et al., J. Org. Chem. 1999, 64,7890-7901; Wagner, R. W. et al., J. Am. Chem. Soc. 1996, 118,11166-11180; Balasubramanian, T.; Lindsey, J. S. Tetrahedron 1999, 55,6771-6784), chlorin building blocks (Strachan, J. P. et al., J. Org.Chem. 2000, 65, 3160-3172), phthalocyanines (Yang, S. I. et al., J.Mater. Chem. 2000, 10, 283-297; Tomoda, H. et al., Chem. Lett. 1980,1277-1280; Tomoda, H. et al., Chem. Lett. 1983, 313-316), and relatedchromophores (Wagner, R. W.; Lindsey, J. S. Pure Appl. Chem. 1996, 68,1373-1380). Methods for joining the chromophore-containing (i.e.,pigment) building blocks into the linear arrays also have beenestablished (Wagner, R. W. et al., J. Org. Chem. 1995, 60, 5266-5273;DiMagno, S. G. et al., J. Org. Chem. 1993, 58, 5983-5993; Wagner, R.W.et al., Chem. Mater. 1999, 11, 2974-2983), which include but are notlimited to Pd-mediated coupling methods. More elaborate architecturescomprised of light-harvesting arrays and a charge-separation unit havebeen synthesized that show very high efficiency (Kuciauskas, D. et al.,J. Am. Chem. Soc. 1999, 121, 8604-8614).

B. Components.

The key requirements for light-harvesting pigments are intenseabsorption in the visible region, a narrow distribution of energies ofthe excited state (marked by sharp absorption and fluorescence bands),an excited singlet-state lifetime sufficient for energy transfer(typically a few nanoseconds), and compatibility with the syntheticbuilding block approach giving rise to a linear architecture. Thepigments of choice for use in the linear LH rods are drawn from theporphyrinic family (tetrapyrrole macrocycles). Examples includeporphyrins, chlorins, bacteriochlorins, tetraazaporphyrins(porphyrazines), phthalocyanines, naphthalocyanines, and derivatives ofthese compounds. The porphyrinic pigments can be supplemented withaccessory pigments such as members of the perylene, lycopene, xanthene,and dipyrromethene families. The absorption spectra of such pigments arewell known to those skilled in the art and can be looked up in variousreference sources (Du, H. et al., Photochem. Photobiol. 1998, 68,141-142).

The important requirements for the linkers joining the pigments are asfollows. (1) Support rapid excited-state energy-transfer processes(through-bond and/or through-space), (2) support ground-statehole-hopping processes in some cases (Designs II and III), and (3)afford compatibility with the synthetic building block approach givingrise to a linear arrangement of pigments. The linkers of choice forjoining the pigments in the linear LH rods include 4,4′-diphenylethyne,4,4′-diphenylbutadiyne, 4,4′-biphenyl, 1,4-phenylene, 4,4′-stilbene,1,4-bicyclooctane, 4,4′-azobenzene, 4,4′-benzylideneaniline,4,4″-terphenyl, and no linker (i.e., a direct C—C bond). Thep,p′-diphenylethyne and p-phenylene linkers have been shown to supportrapid excited-state energy transfer and ground-state hole-hoppingprocesses among porphyrinic molecules.

One important requirement for the charge-separation unit (CSU) is tohave an excited-state of energy equal to or lower than that of theadjacent pigments in the LH array (in other words, absorb light atwavelengths equal to or longer than that of the pigments in the LHarray). For semiconductor based solar cells the excited-state reductionpotential must be greater than the conduction band edge. Additionalrequirements for the CSU are to undergo rapid excited-state electrontransfer, have sufficient energy to inject an electron into theconduction band of the electrode, and afford a stable radical cation.Molecules of choice for the CSU also are drawn from the porphyrinicfamily, including porphyrins, chlorins, bacteriochlorins,tetraazaporphyrins (porphyrazines), phthalocyanines, naphthalocyanines,and derivatives of these compounds. A particularly attractive group ofderivatives is comprised of the double-decker sandwich molecules with acentral metal such as zirconium (Kim, K. et al., Inorg. Chem. 1991, 30,2652-2656; Girolami, G. S. et al., Inorg Chem. 1994, 33, 626-627;Girolami, G. S. et al., Angew. Chem. Int. Ed. Engl. 1996, 35, 1223-1225;Collman, J. P. et al., Inorg. Chem. 1997, 36, 5603-5608). Examples ofdouble-decker sandwich molecules are shown in FIG. 6. Such doubledeckers can be formed from any of the ligands in the family oftetrapyrrole macrocycles.

In the porphyrinic family the electrochemical potential of a givenporphyrin can be tuned over quite a wide range by incorporation ofelectron-withdrawing or electron-releasing substituents (Yang, S. I. etal., J. Porphyrins Phthalocyanines 1999, 3, 117-147). Examples of suchsubstituents include aryl, phenyl, cycloalkyl, alkyl, halogen, alkoxy,alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato,nitro, amino, N-alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, andcarbamoyl. With monomeric porphyrins variation in electrochemicalpotential can also be achieved with different central metals (Fuhrhop,J.-H.; Mauzerall, D. J. Am. Chem. Soc. 1969, 91, 4174-4181). A widevariety of metals can be incorporated in porphyrins. Those metals thatare photochemically active include Zn, Mg, Al, Sn, Cd, Au, Pd, and Pt.It is understood that some metals carry a counterion. Porphyrinsgenerally form very stable radical cations (Felton, R. H. In ThePorphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. V, pp53-126).

The linkers joining the CSU to the electrode surface provide a lineararchitecture, support through-space and/or through-bond electrontransfer, and have a functional group suitable for attachment to theelectrode. Examples of suitable functional groups include ester,carboxylic acid, boronic acid, thiol, phenol, silane, hydroxy, sulfonicacid, phosphonic acid, alkylthiol, etc. The linkers can consist of4,4′-diphenylethyne, 4,4′-diphenylbutadiyne, 4,4′-biphenyl,1,4-phenylene, 4,4′-stilbene, 1,4-bicyclooctane, 4,4′-azobenzene,4,4′-benzylideneaniline, 4,4″-terphenyl, 1,3-phenyl,3,4′-diphenylethyne, 3,4′-diphenylbutadiyne, 3,4′-biphenyl,3,4′-stilbene, 3,4′-azobenzene, 3,4′-benzylideneaniline, 3,4″-terphenyl,etc.

C. Materials.

The innovation of synthesizing chromophoric arrays designed tovectorially translate energy and charge when assembled on electrodesurfaces will allow conductive materials to be used as the substratesfor solar energy conversion devices. This diversity of materials allowsfor the development of designer solar cells for specific applications,some of which are described above. Below the materials and solar energyconversion mechanisms (Designs I-III) that are expected to yieldimproved conversion efficiencies are described.

1. Semiconductor-Chromophoric Array Junctions. Anodic photocurrentgeneration is the most common and efficient mechanism by which solarenergy can be harvested at semiconductors with molecular chromophores(Gerischer, H. Photochem. Photobiol. 1972, 16, 243; Gerischer, H. PureAppl. Chem. 1980, 52, 2649; Gerischer, H.; Willig, F. Top. Curr. Chem.1976, 61, 31). Semiconducting materials such as TiO₂ (rutile oranatase), ZnO, SrTiO₃, SnO₂ and In₂O₃ are thermodynamically stable andcan be processed as thin films, polycrystalline substrates, colloidalparticle thin films, or single crystals with high transparency in thevisible region. The large band gap (>3 eV) assures that directexcitation of the semiconductor will be minimal for terrestrialapplications. Further, materials such as SnO2 are commercially availableon flexible polymeric substrates.

A pictorial representation of a Gerischer-type diagram for the commonlyaccepted mechanism is given in FIG. 7 for a molecular sensitizer, S. Forthe case shown the excited-state reduction potential lies above theconduction band edge by an amount greater than the reorganization energy[E^(o)(S^(+/*))−λ>E_(CB)]. This energetic positioning results in maximumoverlap of the sensitizer excited-state donor levels and thesemiconductor conduction band continuum, which in turn gives rise to themaximum electron-transfer rate, i.e. activationless interfacial electrontransfer. The excess energy of the injected electron is dissipatedthrough lattice vibrations (phonons) as the electron thermalizes to theconduction band edge. Injection of the electron therefore isirreversible, and indeed, repopulation of the excited state has neverbeen observed.

The fate of the injected electron is expected to depend on the biascondition of the semiconductor. If injection occurs when thesemiconductor is near the flat band condition (a in FIG. 7), then rapidrecombination with the oxidized dye is expected because there is noelectric field region to assist the spatial separation of the injectedelectron and oxidized hole. If the semiconductor is under depletionconditions, then the injected electron is swept toward the bulk by thesurface electric field and recombination is inhibited (b in FIG. 7).Therefore, the current onset in photocurrent-voltage measurement can betaken as a crude estimation of the flat band potential. The enhancedlifetime of the charge-separated state at molecular-semiconductorinterfaces under depletion conditions allows the sensitizer to beregenerated by an external electron donor for applications inregenerative solar cells.

A simplified regenerative solar cell based on a molecular sensitizer onan n-type semiconductor is shown schematically in FIG. 8. An excitedsensitizer, S*, injects an electron into the semiconductor, with rateconstant k_(inj). The oxidized dye molecule accepts an electron from adonor (i.e., mobile charge carrier) present in the electrolyte, k_(red).Iodide is the donor shown in the Figure. The iodide oxidation productsare reduced at the dark cathode. The net process allows an electricalcurrent to be generated with light of lower energy than thesemiconductor bandgap with no net chemistry. Charge recombination,k_(cr), can occur to the oxidized sensitizer, S⁺, or an oxidized donorspecies. The donor shown is iodide, which can be dispersed in water,organic solvent, or polymeric gels. Alternative redox activeelectrolytes include Br⁻/Br₂, quinone/hydroquinone, and inorganiccoordination compounds.

Solid-state cells can be used that would replace the redox activeelectrolyte by “hole” conductors, p-type semiconductors, or perhapsmetals. The first two alternatives have precedence and hole conductorssuch as TPD (N,N-diphenyl-N′N′-bis(3-methylphenyl)-1,4-phenylenediamine)or OMeTAD[2,2′,7,7′-tetrakis(N,N-bis(p-methoxyphenyl)amine]-9,9′-spirobifluoreneare known (Salbeck, J. et al., Synth. Met. 1997, 91, 209). p-Typesemiconductors such as CuNCS have been employed in this regard (O'Regan,B.; Schwartz, D. T. Chem. Mater. 1998, 10, 1501). In both cases thematerials must be thermodynamically capable of reducing an oxidizedcomponent of the chromophore array. An example of how such a cell mightwork with a hole conductor is shown in FIG. 9.

By employing chromophoric arrays rather then a single monolayer ofsensitizer molecules S, considerable improvement in efficiencies will berealized. For example, in Design I the sensitizer S is replaced by theCSU to which a rigid array of chromophores is covalently linked. Theentire array extends normal to the electrode surface. The advantage ofthis approach is that the actual surface area occupied on the electrodesurface is comparable to that of a single sensitizer S, but the lightharvesting efficiency is significantly larger. In the solar cellsdescribed the incident photon-to-current efficiency (IPCE) is theproduct of three terms, Equation 1.

IPCE=LHE×Φ_(inj)×η  (1)

The term LHE stands for light-harvesting efficiency and is equivalent tothe IUPAC term α (absorptance), which is equal to the fraction of lightabsorbed (i.e., (1−T) where T is the transmittance). The term Φ_(inj) isthe quantum yield of interfacial electron injection into the electrode.The term η is the electron collection efficiency; that is, the fractionof electrons injected that reach the electrical circuit. The chromophorearrays in Design I will increase the LHE without lowering the otherterms and a higher solar conversion is therefore expected.

Applying Design II to the sensitized semiconductor electrode is expectedto provide increased solar conversion efficiency over Design I, as the‘hole’ will be translated toward the terminal chromophore in the arrayand away from the CSU and the semiconductor electrode. This holemigration will prevent recombination of the electron in thesemiconductor with the ‘hole’ in the chromophoric array. Furthermore,since the regeneration step will occur further from the electrodesurface, decreased recombination with the donor oxidation products isexpected. Both of these kinetic improvements will increase η, theelectron collection efficiency, in Equation 1 and a higher photocurrentis expected. Furthermore, the open circuit photovoltage, V_(oc), isexpected to increase. In regenerative solar cells the maximum Gibbs freeenergy that can be obtained corresponds to the energetic differencebetween the Fermi level of the semiconductor and the redox potential ofthe electrolyte, V_(oc). In preventing recombination of the injectedelectrons the Fermi level is raised and V_(oc) increases. This effectcan be very large. For example, in the Gratzel-type cell, recombinationlosses to the oxidized iodide product only account for the loss ofnanoamps of photocurrent while ˜200 mV of V_(oc) is sacrificed. Sincepower is the product of current and voltage, this represents asignificant loss.

It might even be possible to avoid redox active electrolytes, holeconductors, or p-type semiconductor junctions and simply use thechromophoric array to shuttle the “hole” directly to a metalcounterelectrode as shown in Design III. A related process is envisionedto occur in the “organic semiconductor” films described above and ishighly desirable, because mediating hole transport to thecounterelectrode always wastes potential energy. Quenching of theexcited state of the chromophore arrays by metallic surfaces is ananticipated problem (vide infra). However, since the chromophore arrayis illuminated through a transparent semiconductor and the array ishighly absorbing, very few excited states will be created near themetallic counterelectrode. Accordingly, the availability of linearchromophoric arrays may make it possible to fabricate efficient cellsusing metallic surfaces.

2. Metal-Chromophoric Arrays. There are two possible excited-stateinterfacial electron-transfer processes that can occur from a molecularexcited state, S*, created at a metal surface: (a) The metal accepts anelectron from S* to form S⁺; or (b) the metal donates an electron to S*to form S⁻. Neither of these processes has been directly observed. Thetwo processes would be competitive and unless there is some preference,no net charge will cross the interface. In order to obtain asteady-state photoelectrochemical response, interfacial backelectron-transfer reactions of S⁺ (or S⁻) to yield ground-state productsmust also be eliminated. Energy transfer from an excited sensitizer tothe metal also is thermodynamically favorable and allowed by bothForster and Dexter mechanisms. There exist theoretical predictions andexperimental data describing ‘energy-transfer’ quenching of molecularexcited states by metals. However, these studies involvephotoluminescence measurements and the actual quenching mechanisms,involving electron or energy transfer, remain speculative. Nevertheless,competitive energy-transfer quenching is often invoked to rationalizethe low photocurrent efficiencies measured at sensitized metallicinterfaces. However, there are many reasons to predict poorsensitization yields from metal electrodes (Gerischer, H. Photochem.Photobiol. 1972, 16, 243; Gerischer, H. Pure Appl. Chem. 1980, 52, 2649;Gerischer, H.; Willig, F. Top. Curr. Chem. 1976, 61, 31).

The chromophore arrays described herein are designed to shuttle energytoward the photoanode and holes away from the photoanode (Designs II andIII). This internal rectification should allow preferential injection ofthe electron into the illuminated electrode, as well as hole transferaway from the electrode. In this case, the depletion layer assistedcharge separation that occurs at semiconductor surfaces may not benecessary, because the relative rates of interfacial charge injectionand recombination will give rise to efficient energy conversion. Whilekinetic control of interfacial electron transfer dynamics has previouslybeen suggested as a practical solar conversion scheme, the efficienciesreported are very low with photocurrents in the nanoamp range (Gregg, B.A.; Fox, M. A.; Bard, A. J. J. Phys. Chem. 1990, 94, 1586). Ifsuccessful, the use of metals would allow any conductive substrate to beused for energy conversion. A wide variety of “transparent metals”, i.e.thin metallic films or meshes such as Au, Al, or Pt on transparentsubstrates are available for this application.

D. Synthetic Approaches.

Two distinct approaches are available for preparing the linear LH/CSUrods. One approach involves a stepwise synthesis and the other approachinvolves a polymerization process. Both approaches employ pigmentbuilding blocks bearing at least one and typically two synthetichandles.

One example of the stepwise synthetic approach employs ethyne (E) andiodo-substituted pigment building blocks (FIG. 10). The Pd-mediatedcoupling of an iodo-substituted pigment and a bifunctional pigmentbuilding block bearing an iodo group and a trimethylsilyl-protectedethyne (TMSE) affords the covalently-linked pigment dimer (FIG. 11).Cleavage of the trimethylsilyl-protected ethyne using tetrabutylammoniumfluoride makes possible a second Pd-mediated coupling reaction. In thismanner the linear architecture is constructed, starting from the distalend and proceeding toward the proximal end. The final reaction involvesattachment of the CSU component. The CSU building block bears theattachment group Y required for attachment to the electrode surface.This same method has been employed to prepare ethyne-linkedmultiporphyrin arrays, as described elsewhere herein.

A wide variety of pigment building blocks can be envisaged, as notedelsewhere herein. In addition, multimeric pigment building blocks can beemployed. One example includes a porphyrin dimer bearing a p-iodophenylgroup and a p-[2-(trimethylsilyl)ethynyl]phenyl group. The resultinglight-harvesting array is composed of p-phenylene-linked porphyrindimers that are joined by p,p′-diphenylethyne groups. Dimers for thisbuilding block approach can be prepared in a rational manner as shown inFIG. 12.

The polymerization approach is illustrated in FIG. 13 using the Suzukicoupling method to join pigment building blocks. A pigment buildingblock bearing a carboxy group and a boronic ester group is attached to asolid-phase resin. Many resins are now available; the Wang resin orsimilar resin is particularly attractive. The Wang resin is across-linked polystyrene resin that enables facile attachment ofcompounds bearing carboxylic acids, and detachment is achieved bytreatment with mild acid in organic solvents. The Suzuki coupling isperformed using a mixture of a bifunctional pigment building block and acapping unit, and one of the well known sets of conditions employing Pdcatalysts and ligands that afford high turnovers and high activity. Herethe bifunctional pigment building block bears a boronic ester and aniodo group, while the capping unit bears only an iodo group. Thepolymerization is performed in the presence of the solid-phase. Theaverage length distribution of the linear rod is controlled by the ratioof the bifunctional pigment building block and the capping unit; ingeneral a ratio of 10:1 or so is employed. Following Suzuki coupling,the solid-phase resin is washed to remove unreacted starting materialand coupling byproducts, then the desired product is obtained bycleavage from the resin under standard conditions. With porphyrinicpigments and acidic cleavage conditions, demetalation of themetalloporphyrin is expected and can be redressed by subsequentmetalation. The mixture of oligomers is then fractionated by sizeexclusion chromatography. Note that the synthesis as displayed hasproceeded from the CSU unit outward to the distal end, affording thelinear LH/CSU rod with attached functional group (carboxy in this case)for attachment to the electrode.

The same approach can be taken with chlorin building blocks asillustrated in FIG. 14. A broader list illustrating other pigmentbuilding blocks suitable for Suzuki coupling is shown in FIG. 15. Twonotable examples include a dimeric pigment building block, and amonomeric porphyrin bearing two boron-dipyrromethene dyes. Theboron-dipyrromethene dyes absorb strongly in the blue-green region ofthe solar spectrum, and transmit energy very efficiently to thecovalently attached porphyrin. An example of the synthesis of aporphyrin bearing one iodo group and one boronic ester derivative isshown in FIG. 16. This route uses established methods for formingdipyrromethanes, acylating the dipyrromethane selectively at the 1,9positions, reducing the resulting diketone to thedipyrromethane-dicarbinol, and condensing the dipyrromethane-dicarbinoland a dipyrromethane to form the corresponding porphyrin (Cho, W.-S. etal., J. Org. Chem. 1999, 64, 7890-7901). Subsequent iodination at thelone free meso position affords the desired building block suitable forSuzuki coupling.

Polymerization is not restricted to Suzuki coupling. An example ofmeso,meso-coupling is illustrated in FIG. 17. A porphyrin with one free(unsubstituted) meso position is attached to the solid phase, thentreated to meso,meso-coupling conditions (AgPF₆ or similar oxidant) inthe presence of a mixture of porphyrins in order to elaborate the LH rod(Osuka, A.; Shimidzu, H. Angew. Chem. Int. Ed. Engl. 1997, 36, 135-137;Yoshida, N. et al., Chem. Lett. 1998, 55-56; Nakano, A. et al., Angew.Chem. Int. Ed. 1998, 37, 3023-3027; Senge, M. O.; Feng, X. TetrahedronLett. 1999, 40, 4165-4168). The porphyrin undergoing polymerization hastwo free meso positions, and the porphyrin that serves as the cappingspecies has only one free meso position. The meso,meso-coupled oligomershave strongly split and broadened Soret bands, an attractive feature forachieving absorption across the solar spectrum.

This same polymerization approach can be carried out to yield ameso,meso-linked oligomer bearing an ethyne at the terminus (FIG. 18).This synthesis is performed using a novel linker to attach theethynyl-porphyrin to the solid phase. After cleavage theethyne-substituted meso,meso-linked oligomer can then be subjected to astepwise coupling procedure in order to attach a CSU. An exampleillustrating attachment of a zirconium porphyrin-phthalocyanine is shownin FIG. 19. Alternatively, other pigments can be attached via stepwisecoupling procedures before joining to a CSU. CSU molecules of choiceinclude chlorins, bacteriochlorins, phthalocyanines, naphthalocyanines,or zirconium double-decker sandwich molecules. This same syntheticapproach is particularly attractive for use with chlorin-linkedoligomers (vide infra).

Each of the two synthesis approaches has advantages and disadvantages.The stepwise approach affords an oligomeric product of defined lengthand enables incorporation of different pigments at defined sites.However, the stepwise approach requires substantial syntheticmanipulations, including multiple cycles of deprotection and coupling,to obtain the desired product. The polymerization approach rapidlyaffords linear oligomers of substantial length. However, the oligomersare polydisperse and the synthesis does not afford control overplacement of different pigments in an array. These features cause thetwo approaches to have distinct applications.

For Designs I and II where identical pigments are employed throughoutthe LH rod, the polymerization approach can be employed. For Design IIIwhere the LH/CSU rods need to be of defined and uniform length forplacement between electrode and counterelectrode, the stepwise synthesisapproach must be employed. The only exception would occur when sizefractionation can effect desired length uniformity, as might occur whenrather short oligomers are desired, in which case polymerization can beemployed. For Designs I, II, and III where different pigments are to beincorporated in the LH rod, the stepwise approach must be employed.Combinations of polymerization and stepwise procedures also can beperformed.

Particular examples of porphyrinic macrocycles that may be used asligands to carry out the present invention include compounds of FormulaX, Formula XI, Formula XII, Formula XIII, Formula XIV, Formula XV,Formula XVI, and Formula XVII below (with formulas XII through XVIIrepresenting various chlorins, including bacteriochlorins andisobacteriochlorins).

wherein:

M is a metal, such as a metal selected from the group consisting of Zn,Mg, Pt, Pd, Sn and Al, or M is absent (in which case the ring heteroatoms K³ through K⁴ are substituted with H,H as required to satisfyneutral valency);

K³, K², K³, K⁴, K⁵, K⁶, K⁷, and K⁸ are hetero atoms, such as heteroatoms independently selected from the group consisting of N, O, S, Se,Te, and CH;

S¹, S², S³, S⁴ S⁵, S⁶, S⁷, S⁸, S⁹, S¹⁰, S¹¹, S¹², S¹³, S¹⁴, S¹⁵ and S¹⁶are independently selected substituents that preferably provide a redoxpotential of less than about 5, 2 or even 1 volt. Example substituentsS¹, S², S³, S⁴ include, but are not limited to, H, aryl, phenyl,cycloalkyl, alkyl, halogen, alkoxy, alkylthio, perfluoroalkyl,perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino,acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl.

In addition, each pair of S¹ and and S⁴, S⁵ and S⁶, and S⁷ and S⁸, mayindependently form an annulated arene, such as a benzene, naphthalene,or anthracene, which in turn may be unsubstituted or substituted one ormore times with a substituent that preferably provides a redox potentialof less than about 5, 2 or even 1 volt, such as H, aryl, phenyl,cycloalkyl, alkyl, halogen, alkoxy, alkylthio, perfluoroalkyl,perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino,acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl. Examples of suchannulated arenes include, but are not limited to:

(It being understood that the rings are appropriately conjugated toretain aromaticity of the fused rings); and wherein each substituent S′is independently selected and preferably provides a redox potential ofless than about 5, 2 or even 1 volt. Examples of such substituentsinclude, but are not limited to, H, aryl, phenyl, cycloalkyl, alkyl,halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl,cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl,imido, amido, and carbamoyl. Particular examples of compounds asdescribed above containing annulated arenes are exemplified by FormulasXX-XXIV below.

In addition, S¹ through S¹⁶ may comprise a linking group (—Q—)covalently linked to an adjacent porphyrinic macrocycle of X¹ throughX^(m+1) or a linking group covalently linked to said first electrode. Inone embodiment of the invention, the linking groups of each porphyrinicmacrocycle are oriented in trans; in another embodiment of theinvention, one or more porphyrinic macrocycles contains linking groupsthat are oriented in cis to one another so that the the light harvestingrods contain bends or kinks, or the linker itself is non-linear oroblique.

Examples of porphyrinic macrocycles that contain annulated arenes asdescribed above include, but are not limited to, porphyrinic macrocyclesof Formula XX, XXI, XXIII and XXIV below:

wherein each substituent S′ is independently selected and preferablyprovides a redox potential of less than about 5, 2 or even 1 volt.Examples of such substituents include, but are not limited to, H, aryl,phenyl, cycloalkyl, alkyl, halogen, alkoxy, alkylthio, perfluoroalkyl,perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylarnino,acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl. Again, to linkthe porphyrinic macrocycle to a substrate, or to another compound suchas another porphyrinic macrocycle in the manners described above, theporphyrinic macrocycle will have to contain at least one substituent andpreferably two substituents S′ which is or are a linker, particularly alinker containing a reactive group (where multiple linkers aresubstituted on the ligand, the linkers may be the same or independentlyselected). Such linkers are as described above.

Particular examples of sandwich coordination compounds that may be usedto carry out the present invention have the Formula XXV (fordouble-decker sandwich compounds):

wherein:

M¹ is a metal of the lanthanide series, as well as Y, Zr, Hf, and Bi,and in the actinide series Th and U (radioactive elements such as Pm aregenerally less preferred);

L¹ and L² are independently selected ligands (e.g., porphyrinicmacrocycles); and

Q¹ and Q² may be present or absent and when present are independentlyselected linkers as described above (the linker preferably including aprotected or unprotected reactive group such as thio, seleno or tellurogroup). Preferably, at least one of Q¹ or Q² is present.

It will also be appreciated that each ligand L may be substituted with asingle linker Q, or may be multiply substituted with linkers Q, asexplained in greater detail below. Thus the molecule of Formula XXV maybe covalently linked to an electrode or substrate by at least one of Q¹or Q².

Each ligand L may be further substituted without departing from thescope of the compounds of Formula XI above. For example, and asexplained in greater detail below, ligands may be covalently joined toanother porphyrinic macrocycle, to a ligand of another sandwichcoordination compound, etc.

To link the porphyrinic macrocycle (which may or may not be a componentof a sandwich coordination comound) to a substrate, or to anothercompound such as another porphyrinic macrocycle in the manners describedabove, at least one ligand in the porphyrinic macrocycle will have tocontain at least one and preferably two substituents S¹ through S^(n) orS′ which is a linker, particularly a linker containing a reactive group(where multiple linkers are substituted on the ligand, the linkers maybe the same or independently selected). Such linkers are designated asY—Q— herein, where: Q is a linker, and Y is a substrate, a reactive siteor group that can covalently couple to a substrate, or a reactive siteor group that can ionically couple to a substrate.

Q may be a linear linker or an oblique linker, with linear linkerscurrently preferred. Examples of oblique linkers include, but are notlimited to, 4,3′-diphenylethyne, 4,3′-diphenylbutadiyne, 4,3′-biphenyl,1,3-phenylene, 4,3′-stilbene, 4,3′-azobenzene, 4,3′-benzylideneaniline,and 4,3″-terphenyl. Examples of linear linkers include, but are notlimited to, 4,4′-diphenylethyne, 4,4′-diphenylbutadiyne, 4,4′-biphenyl,1,4-phenylene, 4,4′-stilbene, 1,4-bicyclooctane, 4,4′-azobenzene,4,4′-benzylideneaniline, 4,4″-terphenyl, 3-mercaptophenyl,3-mercaptomethylphenyl, 3-(2-mercaptoethyl)phenyl,3-(3-mercaptopropyl)phenyl, 3-(2-(4-mercaptophenyl)ethynyl)phenyl,3-carboxyphenyl, 3-carboxymethylphenyl, etc.

Y may be a protected or unprotected reactive site or group on the linkersuch as a thio, seleno or telluro group.

Thus, examples of linear linkers for Y—Q— are:4-[2-mercaptoethyl)phenyl, 4-[3-mercaptopropyl)phenyl, an ω-alkylthiolof form HS(CH₂)_(n)—where n=1-20, 4-carboxyphenyl,4-carboxymethylphenyl, 4-(2-carboxyethyl)phenyl, an ω-alkylcarboxylicacid of form HO₂C(CH₂)_(n)—where n=1-20,4-(2-(4-carboxyphenyl)ethynyl)phenyl,4-(2-(4-carboxymethylphenyl)ethynyl)phenyl,4-(2-(4-(2-carboxyethyl)phenyl)ethynyl)phenyl,4-(2-(4-mercaptophenyl)ethynyl)phenyl, 4-mercaptomethylphenyl,4-hydroselenophenyl, 3-(2-(4-hydroselenophenyl)ethynyl)phenyl,4-hydrotellurophenyl, and 4-(2-(4-hydrotellurophenyl)ethynyl)phenyl.

Examples of oblique linkers for Y—Q— are:3-(2-(4-mercaptophenyl)ethynyl)phenyl, 3-mercaptomethylphenyl,3-hydroselenophenyl, 3-(2-(4-hydroselenopenyl)ethynyl)phenyl,3-hydrotellurophenyl, and 3-(2-(4-hydrotellurophenyl)ethynyl)phenyl;etc.

Other suitable linkers include, but are not limited to,2-(4-mercaptophenyl)ethynyl, 2-(4-hydroselenophenyl)ethynyl, and2-(4-hydrotellurophenyl)ethynyl.

Thus, linkers between adjacent porphyrinic macrocycles within a lightharvesting rod, or between a porphyrinic macrocycle and an electrode,are typically those that permit superexchange between the linkedchromophores (mediated electronic communication between chromophoreswhich permits or allows excited-state energy transfer and/or exchange ofelectrons and/or holes). Examples of suitable linkers may be generallyrepresented by the formula —Q—, where Q may be a direct covalent bond ora linking group of the Formula:

R¹—R²_(n)R³—

wherein:

n is from 0 or 1 to 5 or 10;

R³ may be present or absent (yielding a direct covalent bond when R³ isabsent and n is 0); and

R¹, R², and R³ are each independently selected from the group consistingof ethene, ethyne, aryl, and heteroaryl groups (e.g., phenyl, andderivatives of pyridine, thiophene, pyrrole, phenyl, etc., which aryland heteroaryl groups may be unsubstituted or substituted one or moretimes with the same substituents listed above with respect toporphyrinic macrocycles).

The geometry of the linkers with respect to the various chromophores andcharge separation groups in the light harvesting rods can vary. In oneembodiment, at least one of X² through X^(m+1) comprises a meso-linkedporphyrinic macrocycle. In another embodiment, at least one of X²through X^(m+1) comprises a trans meso-linked porphyrinic macrocycle. Inanother embodiment, X² through X^(m+1) consist of meso-linkedporphyrinic macrocycles. In another embodiment, X² through X^(m+1)consist of trans meso-linked porphyrinic macrocycles. In anotherembodiment, at least one of X² through X^(m+1) comprises a β-linkedporphyrinic macrocycle. In another embodiment, at least one of X²through X^(m+1) comprises a trans β-linked porphyrinic macrocycle. Instill another embodiment, X² through X^(m+1) consist of β-linkedporphyrinic macrocycles. In still another embodiment, X² through X^(m+1)consist of trans β-linked porphyrinic macrocycles.

E. Design Examples.

Examples of specific molecules that can achieve the various designs areshown in the following schemes. An example of Design I is shown in FIG.20. The Zn porphyrins constitute the LH rod and the Mg porphyrincomprises the CSU. Energy transfer occurs reversibly among the Znporphyrins but occurs irreversibly (downhill) to the Mg porphyrin(Hascoat, P. et al., Inorg. Chem. 1999, 38, 4849-4853). In this designeach of the Zn and Mg porphyrins has identical non-linking mesosubstituents. Upon charge separation, the hole resides on the Mgporphyrin. The hole cannot transfer to the Zn porphyrins because the Mgporphyrin lies at lower potential than the Zn porphyrins (Wagner, R. W.et al., J. Am. Chem. Soc. 1996, 118, 3996-3997). The oxidation potentialof the CSU can be tuned through alteration of the inductive effect ofthe two non-linking meso substituents. By changing the non-linking mesosubstituents on the Zn porphyrins as well, again the hole is forced toremain on the Mg porphyrin (the CSU).

An example of Design II is shown in FIG. 21. This design is similar tothat shown in FIG. 19 but the non-linking meso substituents on the Znand Mg porphyrins are different. Strongly electron-withdrawingsubstituents are placed on the Mg porphyrin (CSU) but not on the Znporphyrins. Consequently, upon injection of an electron into theelectrode, the hole on the CSU transfers to the Zn porphyrins in the LHarray. Hole transfer is favored because the Zn porphyrins are at lowerpotential than the Mg porphyrin.

A related example of Design II is shown in FIG. 22. Here the linear LHrod is comprised of a series of m perylene components and a series of nZn porphyrins, and the CSU is comprised of a Zn chlorin. Energy flowsirreversibly from the set of perylenes to the Zn porphyrins to the Znchlorin. The hole created at the CSU upon electron injection into theelectrode migrates to the lower potential Zn porphyrins but cannotmigrate into the perylene array. This array is synthesized using aperylene building block bearing an ethyne and an iodo group analogous toporphyrin building blocks.

An additional example illustrating Designs I and II is shown in FIGS. 23and 24. Here β,β′-substituted chlorins are employed in the LH rod, and azirconium double-decker porphyrin-phthalocyanine sandwich moleculeserves as the CSU. Zirconium double-decker porphyrinic sandwichmolecules are known to be photochemically active. The synthesis involvesSuzuki coupling to form the p-phenylene linked chlorin LH rod, whichemploys a linker for attaching an ethyne to the solid phase. Uponcleavage from the solid phase and deprotection, the ethynyl-substitutedLH rod can be attached to the zirconium double decker via a Pd-mediatedethynylation reaction. The zirconium double decker can be obtained byknown methods employing the desired trans-substituted porphyrin. Bychoosing the types of substituents in the porphyrin and phthalocyaninecomponents of the double decker, the oxidation potential can be tuned asdesired. With no electron-releasing substituents, the oxidationpotential is quite low and the hole (formed upon electron injection intothe electrode) resides on the CSU (Design I). With electron-withdrawingsubstituents (e.g., R=F or perfluoroalkyl; Ar¹═Ar²=perfluoroalkyl orpentafluorophenyl) the oxidation potential is increased and the holemigrates to the Zn chlorins in the LH rod (Design II).

An example of Design III is shown in FIG. 25. The LH rod is constitutedwith a series of metallochlorins. The linker for attachment to theelectrode is a benzoic acid derivative, and the linker for attachment tothe counterelectrode is a thioacetate. Such S-acetyl protected thiolsundergo cleavage upon exposure to strong base or upon contact withelectroactive surfaces such as gold (Gryko, D. T. et al., J. Org. Chem.1999, 64, 8635-8647). The LH and CSU components are designed tofacilitate hole migration from the CSU to the distal end where theattachment is made to the counterelectrode.

Another type of pigment that can be used in these designs is abacteriochlorin. Bacteriochlorins absorb strongly in the blue, likeporphyrins, but also absorb strongly in the near-infrared and across thevisible region (e.g., tetraphenylbacteriochlorin, ε_(378 nm)=160,000M⁻¹cm⁻¹, ε_(520 nm)=160,000 M⁻¹cm⁻¹, ε_(740 nm)=130,000M⁻¹cm⁻¹)(Whitlock, H. W. et al. J. Am. Chem. Soc. 1969, 91, 7485). Thestructure of a linear LH rod comprised of bacteriochlorins is shown inFIG. 26.

F. Comparisons of Architectures and Intrinsic Rectification.

A very attractive feature of Designs II and III is that the sequence ofpigments in the light harvesting rod causes energy to flow irreversiblyfrom the site of absorption to the CSU. Simulations show that suchenergy gradients provide a dramatic increase in the quantum efficiencyfor excitation energy reaching the trap (in this case, the chargeseparation unit) (Van Patten, P. G. et al., J. Phys. Chem. B 1998, 102,4209-4216). An added feature of Design III is that the hole (formed atthe CSU) flows irreversibly via hole-hopping from the CSU to thecounterelectrode. The judicious selection of pigments makes possible theirreversible flow of energy and holes in opposite directions in the LHrod. Linear rods that enable energy and holes to flow in oppositedirections are ideally suited for incorporation in the ultrathin solarcells described herein.

The phenomena described above allows these designs to provide intrinsicrectification. Intrinsic rectification can involve (i) the irreversibleflow of excited state energy or electrons along some or all of thelength of the light harvesting rod, (ii) the irreversible flow of holesalong some or all of the length of the light harvesting rod, or (iii)both (i) and (ii) above, with holes and energy or electrons moving inopposite directions.

For irreversible energy migration (intrinsic rectification of energy orelectrons), the light harvesting rod should be structured so thatE*(X¹)<E*(X²)<E*(X³) . . . <E*(X^(n)), where E*(X^(i)) is the energy ofthe excited state of the i^(th) chromophore component X.

For irreversible hole hopping (intrinsic rectification of holes), thelight harvesting rod should be structured so thatE_(½)(X¹)>E_(½)(X²)>E_(½)(X³) . . . >E_(½), where E_(½) is theelectrochemical midpoint oxidation potential of the i^(th) chromophorecomponent X.

While the chromophores of the light harvesting rods could beisoenergetic, the provision of intrinsic rectification as describedabove is preferred. However, intrinsic rectification need not occuralong the entire light harvesting rod, nor even be provided adjacent tothe charge separation unit. For example, one or more isoenergeticchromophores could be provided adjacent to the charge separation unitand intrinsic rectification of holes and/or energy provided elsewherewithin the light-harvesting rod, such as in an intermediate or distalsegment. Thus the term “intrinsic rectification” of excited-stateenergy, holes, and/or electrons by a light-harvesting rod refers tointrinsic rectification that is carried out along any segment or portionof said light harvesting rod.

G. Fabrication of the Solar Cell.

The deposition of the linear LH/CSU molecules on the electrode isperformed by reacting the electrode with a solution of the LH/CSUmolecules, followed by washing to remove any unbound species.Homogeneous or heterogeneous depositions can be performed. That is, ahomogeneous population of molecules can be deposited, or mixtures of theLH/CSU molecules can be employed. One advantage of the latter procedureis that molecules having different components in the LH array can beused to cover the solar spectrum. The various types of LH arrays includebut are not limited to the following: all-chlorin, bacteriochlorin,porphyrin+phthalocyanine, meso,meso-linked porphyrins,perylene+porphyrin arrays. Mixtures of these types of LH arrays can beused in a solar cell to provide effective solar coverage. In thismanner, it is not essential that each LH/CSU rod provide completecoverage of the solar spectrum.

Photogalvanic-like solar cells can be fabricated by positioning a chargeseparation unit away from the electrode surfaces. The incorporation of adriving force for electron transfer to the anode and a driving force forhole transfer to the cathode will result in efficient energy conversion.A potential advantage of this approach is that light harvesting andintramolecular charge separation will occur distant from the electrodesurfaces thereby minimizing deleterious side reactions of the excitedstates and the electrode, such as excited-state quenching.

III. Chlorin Building Blocks for the Construction of Light-HarvestingArrays.

A. Introduction.

Chlorins have three advantages compared with porphyrins for use in solarcollection and utilization. (1) Chlorins absorb strongly both in theblue region and in the red region of the visible region (hence theirgreen color), effectively covering much of the solar spectrum, whileporphyrins absorb strongly only in the blue region (hence their redcolor). (2) The transition dipole moment of the long wavelengthabsorption in a chlorin is linearly polarized along one N-N axis,affording enhanced directionality of through-space energy transfer withneighboring pigments. In contrast, in a metalloporphyrin the transitiondipole moment of the long wavelength absorption band lies in the planeof the macrocycle effectively localized along both N-N axes (planaroscillator), and therefore less directionality of energy transfer isobserved. (3) Chlorins are more easily oxidized than porphyrins andtherefore are better photoreductants.

A synthetic route that provides access to a new set of chlorin buildingblocks has recently been described (Strachan, J. P. et al., J. Org.Chem. 2000, 118, 3160-3172). The chlorin building blocks exhibit typicalchlorin absorption and fluorescence spectra. The chlorin building blockshave substituents (functional handles) at two of the meso positions, andnone at the β positions (FIG. 27). In prior studies of energy transferin multiporphyrin arrays, several findings germane to the design ofpigments for incorporation in light-harvesting arrays comprised ofcovalently-linked pigments were made: (1) The energy transfer processinvolves both through-space (TS) and through-bond (TB) mechanisms, andthe observed rate of energy transfer is the sum of the two processes(Hsiao, J.-S.; et al., J. Am. Chem. Soc. 1996, 118, 11181-11193). (2)The rate of TB energy transfer is affected by the nature of themolecular orbitals in the energy donor, energy acceptor, and linkerjoining the donor and acceptor (Strachan, J. P. et al., J. Am. Chem.Soc. 1997, 119, 11191-11201; Yang, S. I. et al., J. Am. Chem. Soc. 1999,121, 4008-4018). In particular, more extensive electronic communication(and a faster rate of transfer) occurs upon attachment of the linker atsites on the donor and acceptor where the frontier molecular orbitalshave high electron density versus sites with low electron density. To beexplicit, in a porphyrin having an a_(2u) HOMO (which has electrondensity predominantly at the meso positions and little at the ppositions), faster rates (2.5-10-fold) are observed with linkers at themeso rather than positions (FIG. 28). Conversely, in a porphyrin havingan a_(1u) HOMO (which has electron density predominantly at the βpositions and little if any at the meso positions), faster rates areobserved with linkers at the p rather than meso positions. Such a ratedifferential is incurred with each pigment-to-pigment energy-transferstep in linear multipigment arrays, which is manifest as a large effecton the overall rate and yield of energy transfer (Van Patten, P. G. etal., J. Phys. Chem. B 1998, 102, 4209-4216).

The factors that affect TS energy transfer (Forster mechanism) are wellknown. Two key determinants are the oscillator strength of theexcited-state donor (reflected in the radiative rate for the lowestenergy transition) and the oscillator strength of the ground-stateacceptor (reflected in the molar absorption coefficient of the lowestenergy transition). Chlorins are ideal candidates for TS energy transferdue to their strong oscillator strength of the long-wavelengthabsorption band, especially compared with porphyrins due to their weakoscillator strength (i.e., weak absorption) in the red. Another keydeterminant involves the orientation of the respective transition dipolemoments of the donor and acceptor. The orientation term (κ²) in ForsterTS energy transfer takes on values of 0 (orthogonal), 1 (parallel butnot collinear), and 4 (collinear) depending on the vector orientation ofthe transition dipole moments. The most efficient energy transfer occurswith molecular arrangements such that the donor and acceptor transitiondipole moments are collinear, and the least efficient occurs withorthogonal orientations (Van Patten, P. G. et al., J. Phys. Chem. B1998, 102, 4209-4216).

The prediction of the energy-transfer properties of chlorin-containingarrays can be summarized. In diphenylethyne linked multiporphyrin arrays(meso-linked, a_(2u) HOMO), the observed rate of energy transfer from azinc porphyrin to a free base porphyrin was found to be (24 ps)⁻¹ withcontributions of (720 ps)⁻¹ for TS transfer and (25 ps)⁻¹ for TBtransfer. For chlorins the radiative rate constant is increased by˜4-fold and the molar absorptivity is increased by ≧10-fold vs.porphyrins. Considering the incorporation of chlorins in an idealgeometry for Forster transfer with the same intervening diphenylethynelinker, the orientation term (κ²) would be up to ˜2 vs. 1.125 forporphyrins. The net result is an expected increase of up to 100-fold inrate of TS energy transfer with chlorins vs. porphyrins.

The TB transfer rate with chlorins is difficult to estimate but shouldfall into the range observed with the porphyrins, which ranged from (25ps)⁻¹ to (360 ps)⁻¹ for depending on orbital density at the site oflinker connection. The TS transfer for chlorins should be 40-100 foldfaster than for porphyrins (i.e., from (18 ps)⁻¹ to ˜(7 ps)⁻¹).Accordingly, the anticipated rate for chlorin-chlorin energy transfershould be in the range of ˜(10 ps)⁻¹ to ˜(20 ps)⁻¹.

In moving from diphenylethyne-linked multiporphyrin arrays top-phenylene linked arrays the observed energy transfer rate increasedfrom (24 ps)⁻¹ to (2 ps)⁻¹. Upon going to a p-phenylene linker withchlorins, the shorter distance should cause an increase of ˜100-fold inthe TS contribution to the rate. In this case, the TS mechanismdominates the TB rate. Accordingly, energy-transfer rates from chlorinto chlorin in p-phenylene-linked arrays are anticipated to be in thesub-picosecond regime for both meso- and β-substituted chlorins.

These rough calculations illustrate that chlorins are anticipated tohave a large TS component of the observed energy transfer process inmulti-chlorin arrays. The faster rates of energy transfer with chlorinsvs. porphyrins, in conjunction with the superior spectral properties(i.e., blue and red absorption) of chlorins vs. porphyrins, providesstrong impetus for constructing chlorin-containing light-harvestingarrays. Both meso-substituted chlorin building blocks and β-substitutedchlorin building blocks are sought.

B. Molecular Design.

Here chlorin building blocks designed to give efficient energy transferin chlorin-containing light-harvesting arrays are presented. Objectivesare to (1) prepare chlorins with two functional handles such that thechlorins can be readily incorporated into linear arrays, (2) design thechlorin building blocks to have the highest possible value of theorientation term for TS energy transfer, and (3) be connectedappropriately to give the most extensive TB energy transfer process.What are the best sites on the chlorin for connection of the linker?Four possible trans-substituted chlorins are displayed in FIG. 29. Twoβ,β′-substituted chlorins are shown, as are two chlorins each bearingtwo meso substituents. To evaluate the chlorin building blocks one mustconsider (1) steric effects of any substituents, (2) the orientation ofthe transition dipole moment for the long wavelength transition, and (3)the composition of the frontier molecular orbitals.

Examination of the four chlorin building blocks in FIG. 29 revealssteric hindrance in chlorin IV due to the interaction of the mesosubstituent flanking the geminal methyl groups of ring D. The otherthree chlorins I-III have no such steric interactions and are superiorto IV in this regard.

The transition dipole moment for the far-red transition in chlorins ispolarized along the N-N axis perpendicular to the reduced ring (ring D),transecting rings A and C not rings B and D (FIG. 30). Evaluation of thefour possible trans-chlorins shown in FIG. 29 requires consideration ofthe geometries obtained upon incorporation in covalently linked arrays.The pairwise interactions are displayed in FIG. 31, where adiphenylethyne linker is employed to join the chlorins (other linkers,including a p-phenylene group could also be employed). For themeso-linked chlorins, κ² takes on limiting values of 0.25 and 2.25depending on orientation. Assuming free rotation during the lifetime ofthe excited state (dynamical averaging), the average value of κ² is1.125. Note that free rotation is expected about the cylindricallysymmetric ethyne but the rate of rotation may not be sufficient to causeall molecules to explore all conformations during the few ns lifetime ofthe excited state. Thus, those molecules in an orientation characterizedby a zero or near-zero value of κ² will not give rise to efficient TSenergy transfer. The β,β′-substituted chlorins have limiting K² valuesof ˜1.6 and the value remains >1 regardless of dihedral angle about theethyne linker. (Note that in this case the center-to-center distancechanges slightly upon rotation about the ethyne linker.) Thus, theβ,β′-substituted chlorins give slightly better collinearity of thetransition dipole moment with the axis of substitution (to be the linearaxis of the multi-chlorin array) than is obtained with themeso-substituted chlorins. Taken together, chlorin building blocks I andII are slightly preferred over III and IV for TS energy transfer.

The highest occupied molecular orbital of a chlorin is an a₂ orbital,which places electron density at each of the meso and β sites (FIG. 32).Accordingly, it is difficult to estimate the relative goodness of mesoversus β sites of linker attachment for efficient TB energy transfer. Inthe absence of this knowledge, the β-substitution chlorins and themeso-substituted chlorins are believed to have comparable utility. Inany event, as the distance of separation of the rings becomes quiteshort, the TS mechanism will dominate and the TB mechanism will become arelatively minor contributor to the observed rate. The chiefdisadvantage of the meso-substituted trans chlorins stems from possibleruffling of the macrocycle due to steric congestion with the partiallysaturated ring. The trans configuration can be achieved with connectionto rings A and C. Comparing the four possible trans-chlorins shown inScheme 4 for TB energy transfer, it is seen that the meso-substitutedchlorins (III, IV) are inferior to the ββ′-substituted chlorins (I, II).

In summary, the chlorin building blocks I, II, and III are useful forconstructing light-harvesting arrays. From a synthetic standpoint,chlorin building block I is more readily accessible than II. Note thatthe previous set of chlorin building blocks that were prepared providedtypical chlorin absorption properties but were inappropriate forpreparing linear arrays having synthetic handles at two adjacent (cis)meso positions rather than two trans meso or β positions (FIG. 27). Incontrast, chlorins I and II are ideal components of syntheticlight-harvesting arrays and should give fast rates of transfer.

C. Synthesis.

The synthesis of the β-substituted chlorin building blocks (type I)follows the general route previously established for preparingmeso-substituted chlorins, as discussed above. In this route, an Easternhalf and a Western half undergo condensation followed by oxidativecyclization to give the chlorin. The same approach is used here with anew Eastern half and a new Western half, each bearing one β substituent(FIG. 33).

The synthesis of the new β-substituted Eastern half is shown in FIGS.34A and 34B. This synthesis, which builds on prior work in developing aroute to β-substituted porphyrin building blocks (Balasubramanian, T.;Lindsey, J. S. Tetrahedron 1999, 55, 6771-6784), is described in Example1 below.

The synthesis of the new β-substituted Western half is shown in FIG. 35.This route begins with the same critical intermediate as used in theEastern half, a 2-formyl-3-arylpyrrole (FIG. 34A). The Western half isthen prepared following the same sequence of reactions employed for theunsubstituted Western half. This latter route is under examination.

The chlorin building block shown in FIG. 33 bears one4-(TMS-ethynyl)phenyl group and one 4-iodophenyl group. This particularbuilding block should enable the synthesis of diphenylethyne linkedchlorin containing arrays in a linear architecture. Other chlorinbuilding blocks that are accessible via this same synthetic strategy,and that have the same desirable physical properties, are shown in FIG.36.

The synthesis of the trans meso-substituted chlorin building blocks(type III) is obtained in two ways. One route involves an extension ofthe route recently established for preparing chlorins bearing adjacent(cis) meso-substituted chlorins (FIG. 27) (Strachan, J. P. et al., J.Org. Chem. 2000, 118, 3160). Treatment of a cis meso-substituted chlorinwith NBS (DiMagno, S. G. et al., J. Org. Chem. 1993, 58, 5983-5993) isanticipated to give selective bromination as shown in FIG. 37.Alternatively iodination can be performed using iodine and AgPF₆(Nakano, A. et al., Tetrahedron Lett. 1998, 39, 9489-9492). Woodwarddemonstrated that the two methine positions flanking ring D are highlyreactive toward electrophilic reagents (Woodward, R. B.; Skaric, V. J.Am. Chem. Soc. 1961, 83, 4676-4678). The position between rings A and Dis sterically hindered by the geminal methyl substituents, which shouldbe sufficient to give reaction selectively at the methine positionssbetween rings D and C. Subsequent Pd-mediated cross-coupling (DiMagno,S. G. et al., J. Org. Chem. 1993, 58, 5983-5993) then gives the desiredtrans substituted (Ar², Ar³) chlorin building block.

A second route to trans meso-substituted chlorin building blocks (typeIII) is shown in FIG. 38. A new Western half is prepared that bears,attached to the partially saturated ring, the substituent destined to bethe corresponding meso substituent.

Various meso-substituted chlorin building blocks that can be accessed inthis manner, and that are useful for incorporation in syntheticlight-harvesting arrays, are shown in FIG. 39. Note that substituents tobe employed as the aryl unit (Ar) can be used to tune theelectrochemical potential, for solubility purposes, or to controlpacking of the light-harvesting arrays in self-assembled structures.

For all of the chlorin building blocks, a wide variety of metals can beemployed, given that the metals meet the requirement of affording aphotochemically active excited state. Preferred embodiments of suchmetals are Zn, Mg, Pd, Sn, and Al. The free base chlorin (M=H, H) canalso be employed. In the syntheses employed, the chlorin-formingreaction yields the zinc chlorin, which is easily demetalated with mildacid to give the free base chlorin. The desired metallochlorin can thenbe prepared via well known metalation reactions.

IV. The Flow of Excited-state Energy and Ground-state Holes in OppositeDirections in Light Harvesting Arrays

A. Introduction.

Sunlight is dilute and therein lies one of the major challenges indeveloping efficient means of utilizing sunlight as a source of energy.The strategy employed by photosynthetic organisms is to absorb sunlightwith multipigment antenna complexes and then funnel the resultingexcited-state energy among the pigments such that the excitation reachesa reaction center (i.e., a charge separation unit). In the reactioncenter a charge separation reaction occurs, giving a reducing equivalent(electron) and an oxidizing equivalent (hole). In plants, the reducingand oxidizing equivalents are ultimately used to reduce carbon dioxide(forming carbohydrates) and oxidize water (liberating oxygen),respectively. Thus, electrons flow both from the reaction center and tothe reaction center (filling the hole created by charge separation). Thereaction center therefore must have three channels; a channel for theflow of excitation energy from the antenna, a channel for the emanationof electrons following charge separation, and a channel for the input ofelectrons to regenerate the reaction center following charge separation(FIG. 40). Note that the inward flow of electrons and the outwardmigration of holes are equivalent processes and these terms are usedsynonymously.

In terms of size, the antenna complexes dwarf the reaction centers.While chlorophyll molecules are contained both in the antenna complexesand in the reaction center, the bulk of the chlorophyll is located inthe antenna complexes. For example, about six chlorophylls (orchlorophyll derivatives) typically reside in a bacterial photosyntheticreaction center, whereas up to several hundred chlorophylls can bepresent in the antenna complexes. The antenna complexes serve to collectdilute sunlight, and the reaction centers initiate the transduction ofexcited-state energy into chemical fuel via the intermediacy of astabilized charge-separated state.

The generation of stable separated charges in the reaction centerinvolves a series of electron transfer steps among a series of electronacceptors. With each step the overall reverse (i.e., recombination) rateof electron transfer becomes slower. After three steps the ratedifferential of the initial forward step (k_(f)) and the rate ofrecombination (k_(recomb)) is given by k_(f)/k_(recomb) ˜10⁶. The seriesof fast forward transfers and slow reverse transfers affords rapid andefficient separation of the electron and hole over a long distance.

A large effort has been devoted to the development of synthetic antennamolecules and synthetic charge-separation units (i.e., the equivalent ofthe reaction center). In general the antennas prepared to date arecomprised of 10 or fewer pigments (Li, J.; Lindsey, J. S. J. Org. Chem.1999, 64, 9101-9108). Other molecules have been constructed that providesmall antennas attached to a charge-separation unit (CSU) (Kuciauskas,D. et al., J. Am. Chem. Soc. 1999, 121, 8604-8614). These molecules havedemonstrated efficient light harvesting (i.e., light absorption andenergy migration) and efficient charge separation. For the most part,the synthetic antennas, CSU, and integrated antenna-CSU systems havebeen studied in solution. Such studies can provide deep insight intomechanisms and properties but rarely address issues for organizing acollection of light-harvesting antennas and CSUs, as must be done forconstructing any practical system for utilizing sunlight.

Light-harvesting arrays that absorb light and undergo efficientintramolecular energy transfer have been designed and synthesized.Examples of such molecules are shown in FIG. 41. With a diphenylethynelinker, the rate of energy transfer from zinc porphyrin to free baseporphyrin is (24 ps)⁻¹. With a p-phenylene linker, the rate of energytransfer from zinc porphyrin to free base porphyrin is (2 ps)⁻¹. Inorder to probe the role of the linker in mediating excited-state energytransfer among the porphyrins, examined the electrochemical propertiesof these multiporphyrin arrays was examined. The electrochemicalpotentials of the individual pigments are maintained upon incorporationinto the arrays. However, the hole created in the multiporphyrin arrayis delocalized as determined by EPR analysis. The rate of hole-hoppingamong porphyrins in the array in fluid solution is faster than can beresolved by the EPR technique, implying a rate >10⁷ s⁻¹ (Seth, J. etal., J. Am. Chem. Soc. 1994, 116, 10578-10592; Seth, J. et al., J. Am.Chem. Soc. 1996, 118, 11194-11207). These studies revealed that thereexist significant ground-state electronic interactions in themultiporphyrin arrays and that the interaction must be mediated by thelinker that joins the porphyrins. In summary, the multiporphyrin arrayshave the desired features of light absorption and excited-state energymigration that are essential for efficient light-harvesting, and inaddition, have the unexpected feature of facile ground-statehole-hopping processes upon formation of the oxidized complexes.

B. Design of Linear Arrays.

One of the challenges in designing a solar cell involves integrating thevarious components, which include an antenna, CSU, and pathways forelectron flow from and to the CSU. Here a novel means of moving theoxidizing equivalent away from the charge-separation unit is proposed.In essence, the antenna is designed such that energy flows along thelight-harvesting array to the charge-separation unit, while theoxidizing equivalent (hole) flows in the reverse direction from the CSUto a site in the antenna where subsequent electron-transfer reactionscan take place (FIG. 42). This design has two significant ramifications.(1) Only two access channels are required at the CSU: one for emanationof the electrons, and one where excitation energy flows in and oxidizingequivalents (holes) flow out (FIG. 43). The existence of two channelsrather than three eases the 3-dimensional packing constraints fororganizing light-harvesting antennas around a CSU. (2) The migration ofthe hole away from the charge-separation unit results in stabilizationof the charge-separated state. The vast majority of approaches exploredfor stabilizing the charge-separated state in synthetic systems hasfocused on the use of a series of electron acceptors for moving theelectron away from the hole. The converse approach (described herein)employs a series of hole acceptors to move the hole far from theelectron and thereby give a stable charge-separated state.

Porphyrinic molecules are central to this design. Porphyrins stronglyabsorb light, and give stable radical cations upon oxidation. Acharacteristic feature of porphyrins is that changes in the nature ofelectron-withdrawing or electron-releasing substituents attached to aporphyrin cause commensurate changes in the electrochemical potentialsof the porphyrin, but do not significantly alter the absorption spectrumof the porphyrin (Seth, J.; Palaniappan, V.; Wagner, R. W.; Johnson, T.E.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1996, 118,11194-11207). Hence, the electrochemical potentials of porphyrins can betuned without changing the energy levels that play a role in energymigration. Said differently, the substituents shift the energy levels ofboth the HOMO and LUMO, causing changes in the oxidation and reductionpotentials, respectively. The absorption spectrum depends on thedifference between the energy of the HOMO and LUMO; if both HOMO andLUMO are shifted identically, no difference in the HOMO-LUMO gap isobserved, and therefore the absorption spectrum remains unchanged. Theability to tune the oxidation potential without affecting the absorptionspectrum makes possible the design of hole-transfer cascades whilemaintaining the energy-transfer properties of the arrays. The judiciouschoice of pigments with different absorption spectra, in conjunctionwith structural modifications for electrochemical tuning, allows energyand holes to flow downhill in opposite directions.

The following examples illustrate the opposite flow of excitation energyand ground-state holes. In FIGS. 44-49, the energy level diagrams areillustrative.

(1) A linear array of zinc porphyrins bearing different mesosubstituents (FIG. 44). The four porphyrins have essentially identicalabsorption spectra, hence energy migration occurs among fourisoenergetic porphyrins. The transfer of energy occurs reversibly amongthe four porphyrins. The electrochemical potentials are arranged in acascade, with the highest potential proximal to the CSU and the lowestpotential distal to the CSU. Hence hole migration occurs irreversibly inmoving from high to low potential.

(2) A linear array of Mg and Zn porphyrins bearing different mesosubstituents (FIG. 45). Mg porphyrins absorb at slightly longerwavelength (˜5-10 nm) than Zn porphyrins and are more easily oxidized(i.e., lower potential) than Zn porphyrins (Li, F. et al., J. Mater.Chem. 1997, 7, 1245-1262; Hascoat, P. et al., Inorg. Chem. 1999, 38,4849-4853). The sequence of Zn, Zn, Mg, Mg in going toward the CSUresults in reversible energy transfer among the two isoenergetic Znporphyrins, irreversible energy migration from Zn to Mg porphyrins, andreversible energy transfer among the two isoenergetic Mg porphyrins.Though Mg porphyrins are more easily oxidized than Zn porphyrins (withidentical substituents), the placement of strongly electron-withdrawingsubstituents on Mg porphyrins and electron-releasing substituents on Znporphyrins causes reversal of the ordering of the oxidation potentials(Yang, S. I. et al., J. Porphyrins Phthalocyanines 1999, 3, 117-147).For the linear array of porphyrins shown in FIG. 45, the arrangement ofsuch substituents causes a cascade from high potential proximal to theCSU to low potential distal to the CSU. Thus, this system affords apartial cascade for energy migration and a stepwise cascade for holemigration in the opposite direction.

(3) A linear array of metallochlorins bearing different mesosubstituents (FIG. 46). Chlorins absorb strongly both in the blue and inthe red regions, effectively covering much of the solar spectrum.Chlorins are members of the porphyrinic (i.e., cyclic tetrapyrrole)family. As with porphyrins, the electrochemical potentials of chlorinsare altered in a rational and predictable manner by the presence ofelectron-withdrawing or electron-releasing substituents. However, theabsorption spectra are essentially unaffected by the inductive effectsof substituents. Thus, the arrangement of chlorins in the linear arrayresults in reversible energy migration among isoenergetic pigments butirreversible hole transfer in moving away from the CSU.

(4) A linear array of porphyrins and chlorins bearing different mesosubstituents (FIG. 47). The long-wavelength absorption band (andtherefore the energy of the excited singlet state) of a metalloporphyrinfalls at shorter wavelength (higher energy) than that of thecorresponding metallochlorin. On the other hand, a chlorin is morereadily oxidized (lower potential) than the correspondingmetalloporphyrin. An energy cascade and a hole cascade can be created asshown in FIG. 47. Strongly electron-withdrawing substituents areemployed with chlorins and electron-releasing substituents are employedwith the porphyrins, shifting the chlorins to higher potential than theporphyrins. Reversible energy transfer occurs among the pair ofporphyrins, irreversible transfer occurs from porphyrin to chlorin, andreversible transfer occurs among the pair of chlorins. Reversible holetransfer occurs among the pair of chlorins, followed by irreversiblehole transfer from chlorin to porphyrin and again from porphyrin toporphyrin.

(5) A linear array of β-substituted chlorins and meso-substitutedchlorins (FIG. 48). The β-substituted chlorins that are currentlyavailable do not bear a third substituent for tuning the electrochemicalpotential, as do the meso-substituted chlorins. Still, the β-substitutedchlorins can be incorporated fruitfully as shown in FIG. 48. Thisarrangement affords reversible energy migration and a cascade of holetransfer processes. Note that the chlorins can be arranged with thepartially saturated ring pointed toward or away from the CSU; nodifferences in performance are implied by the different orientationsdisplayed in FIG. 48.

(6) A linear array of porphyrin, chlorin, and phthalocyanine components(FIG. 49). Phthalocyanines are characterized by very strong absorptionin the red (ε˜250,000 M⁻¹cm⁻¹) and high oxidation potentials (Li, J. etal., J. Org. Chem. 1999, 64, 9090-9100; Yang, S. I. et al., J. Mater.Chem. 2000, 10, 283). The array shown in FIG. 49 exhibits a progressiveenergy cascade from Zn porphyrin to Mg porphyrin to chlorin tophthalocyanine. The hole migration process occurs in the oppositedirection, with tuning of the Mg chlorin and Zn porphyrin using mesosubstituents.

C. Other Compositions.

The porphyrinic family includes diverse pigments, many of which can beemployed in linear architectures for energy and hole migration inopposite directions. Noteworthy members include tetraazaporphyrins,heteroatom-modified porphyrins (e.g., N₃O— or N₃S— instead of thestandard N₄-porphyrins) (Cho, W.-S. et al., J. Org. Chem. 1999, 64,7890-7901), corrole, and various expanded and contracted porphyrins (VanPatten, P. G. et al., J. Phys. Chem. B 1998, 102, 4209-4216).

In general, a wide variety of substituents are available for tuningelectrochemical potentials, including diverse aryl and alkyl groups.Halogenated aryl groups are particularly attractive for fine tuning theelectrochemical potentials.

The arrays displayed in FIGS. 44-49 all employ diphenylethyne orp-phenylene linkers. The energy-transfer rate in going from adiphenylethyne linker to a p-phenylene linker increases by about10-times for porphyrins (˜(2 ps)⁻¹)⁵ and 100-times for chlorins(sub-picoseconds). While these linkers are quite attractive, otherlinkers also can be used.

D. Other Designs.

The availability of rapid energy-transfer processes (i.e., withp-phenylene linked chlorins or porphyrins) has important consequencesfor the design of light-harvesting arrays. Simulations of the effects ofenergy-transfer rates on the quantum yield for energy transfer to thetrap positioned at the end of a linear array showed the following (VanPatten, P. G. et al., J. Phys. Chem. B 1998, 102, 4209-4216): Withreversible transfer among isoenergetic pigments, the quantum yield fallsoff very steeply with number of pigments. However, the quantum yield isalso very sensitive to the rate of transfer. An increased rate, even forreversible transfer among isoenergetic pigments, mitigates the falloffin quantum yield with increasing numbers of pigments. With rates in thefew ps to sub-ps regime, linear arrays of a reasonable number ofisoenergetic pigments (e.g., up to 20) should afford acceptable quantumyields for excitation reaching the trap (i.e, the CSU). Conversely, withslow transfer rates (tens of psec), a high quantum yield in a modestlysized linear array can be obtained only by using an energy cascade(i.e., irreversible energy transfer steps). In summary, a linear arraycomprised of several identical (iosoenergetic) pigments will affordefficient energy transfer if the rate of transfer is very rapid.

An example of an array employing multiple isoenergetic pigments isillustrated in FIG. 50. This array incorporates the same Zn porphyrin,Mg porphyrin, and Zn chlorin pigments as employed in FIG. 49. Thelinkers are exclusively p-phenylene groups. The architecture employs aset of n Zn porphyrins, then a set of n Mg porphyrins, and then one Znchlorin. Energy and hole transfer occur reversibly among members of agiven set, but transfer occurs irreversibly (downhill) between sets.This arrangement of pigments resembles a type of cataract architecture.This design illustrates the concept that it is not necessary to have adownhill energy transfer step occur at each pigment in order to achieveefficient energy transfer from the site of light absorption to the CSU.

For rapid energy transfer, ideal choices are to use p-phenylene linkerswith porphyrins (having a_(2u) HOMOs and linkers at the meso positions)(Yang, S. I. et al., J. Am. Chem. Soc. 1999, 121, 4008-4018) and/orchlorins (having linkers at the meso or β positions). For rapidhole-hopping, the linker should be connected at a site on the pigmenthaving significant electron density in the HOMO. While energy transfercan proceed via TS and/or TB mechanisms, ground-state hole-hoppingproceeds exclusively via a TB mechanism (at least for the rapid ratesobserved). The factors that affect TB energy migration are similar tothose that affect ground-state hole-hopping. Thus, those arrays withfast TB energy migration should also give rapid rates of hole-hopping.For efficient hole-hopping, it also is not essential to have a downhillhole-hopping step occur at each pigment in order to achieve efficienttransfer from the CSU to a site far removed in the light-harvestingantenna.

Although in these designs excited-state energy and ground-state holesare formed in the same array, given the rapid dynamics of energymigration and the low flux of ambient sunlight, the probability is quitelow for the simultaneous presence of an excited state and a hole.Accordingly, excited-state quenching by ground-state holes isanticipated to be an infrequent occurrence.

V. In Situ Synthesis of Light-harvesting Polymer on ElectroactiveSurfaces

The synthesis of oligomers of pigment building blocks (BB), orlight-harvesting rods, can proceed via several different types ofreactions. A general issue is that the reaction used to join the pigmentbuilding blocks into a dyad architecture also creates the linker thatprovides electronic communication between the two pigments. Accordingly,a more limited set of reactions is generally envisaged than that in theentire corpus of organic chemistry. The methods for synthesis ofpolymeric arrays of pigment building blocks include but are notrestricted to use of the following types of reactions (FIG. 51):

Glaser (or Eglinton) coupling of a monomeric pigment building blocks(generating a butadiyne linker)

Cadiot-Chodkiewicz coupling of two different pigment building blocks(generating a butadiyne linker joining a block copolymer)

Sonogashira coupling of two different pigment building blocks(generating an ethyne linker joining a block copolymer)

Heck or Witting reactions of two different pigment building blocks(generating an alkene linker joining a block copolymer)

Suzuki coupling of two different pigment building blocks (generating aphenylene or biphenyl linker joining a block copolymer)

Polymerization of pigment building blocks bearing substituents such astwo or more thiophene groups (generating an oligothiophene linker) ortwo or more pyrrole groups (generating a polypyrrole linker).

The synthesis of the oligomers can be performed using stepwise methodsor using polymerization methods. Both methods generally require tworeactive groups attached to the pigment building block in order toprepare a polymer where the pigment building blocks are integralcomponents of the polymer backbone. (An alternative, less attractivedesign yields pendant polymers where the pigment building blocks areattached via one linkage to the polymer backbone.) The stepwisesynthetic method generally requires the use of protecting groups to maskone reactive site, and one cycle of reactions then involves couplingfollowed by deprotection. In the polymerization method no protectinggroups are employed and the polymer is prepared in a one-flask process.

The polymerizations can take place in solution or can be performed withthe polymer growing from a surface. The polymerization can be performedbeginning with a solid support as in solid-phase peptide or DNAsynthesis, then removed, purified, and elaborated further for specificapplications. The polymerization with the nascent polymer attached to anelectroactive surface generates the desired light-harvesting material insitu. This latter approach is exceptionally attractive in eliminatingthe need for handling of the polymers. The ability to avoid handling ofthe polymers makes possible the synthesis of compounds that do notexhibit sufficient solubility in most solvents for convenient handling(dissolution, purification, processing, solution characterization).

Polymers can be created that are composed of identical units, ordissimilar units as in block copolymers or random copolymers.Alternatively, the polymerization can be performed to create a lineararray where the composition of different pigment building blocks isorganized in a gradient. This latter approach affords the possibility ofcreating an energy cascade for the flow of excited-state energy and/orthe reverse flow of ground-state holes in a systematic manner along thelength of the array as described elsewhere in this application.

The following describes the in situ synthesis of the cascade polymers onan electroactive surface such as gold or TiO₂: A polymerizable unit(pigment building block or linker) is attached to the surface (for Au, athiol attachment group is used for Y¹; for TiO₂, a carboxylic acidattachment group is used for Y²). The first pigment building block (BB¹)is added and the coupling reagents are added in order to perform thepolymerization (e.g., a Glaser coupling). Then the surface is washed toremove the coupling reagents (copper reagents in the case of the Glasercoupling) and any unreacted BB¹. Then the second pigment building block(BB²) is added followed by coupling reagents and the polymerization isallowed to continue. The same wash procedure is performed again and thenthe third pigment building block (BB³) is added followed by couplingreagents and the polymerization is allowed to continue. Repetition ofthis process enables the systematic construction of a linear array ofpigment building blocks with graded energy levels for the flow ofexcited-state energy and ground-state holes. The last monomer attachedbears a single reactive site (J—L—(BB)—L—Y) and the attachment group Yis used for subsequent coupling to an opposing surface. Characterizationof the surface-immobilized polymer is achieved by absorptionspectroscopy, IR spectroscopy, reflectance spectroscopy and laserdesorption time of flight mass spectrometry.

In the example shown (see FIG. 52), for a surface of Au, a thiolattachment group (X) is used, creating the self-assembled monolayer ongold. Such self-assembled monolayers are known for thiol-derivatizedporphyrins (Gryko, D. T. et al., J. Org. Chem. 1999, 64, 8635-8647). Forthe other surface composed of TiO₂, a carboxylic acid attachment groupis used for the attachment (Y). The polymerizable groups can be any ofthe type described above using the various name reactions (Glaser,Sonogashira, Cadiot-Chodkiewicz, Heck, Wittig, Suzuki, etc.). The finalpolymeric product is comprised of domains of the various pigmentbuilding blocks [(BB^(i))_(n)] in a linear array.

VI. Applications of Solar Cells of the Invention

Solar cells of the present invention can be used in a variety ofdifferent electrical devices. Such devices typically comprise a solarcell as described above, and a circuit (e.g., a resistive load)electrically coupled to said solar cell (e.g., by providing a firstelectrical coupling of the circuit to one electrode of the solar cell,and a second electrical coupling of the circuit to the other electrodeof the solar cell). The solar cell may provide the sole source of powerto the circuit, may be a supplemental source, may be incorporated tocharge a battery, etc. Any of a variety of different electrical devicesmay incorporate a solar cell of the invention, including but not limitedto radios, televisions, computers (such as personal computers),processors, calculators, telephones, wireless communication devices suchas pagers, watches, emergency location devices, electric vehicles,emergency power supplies, power generators, lights or lamps, and otherilluminating devices, monitoring devices, inspection devices, radiationdetectors, imaging devices, optical coupling devices.

The following examples are provided to illustrate certain aspects of theinvention, and are not to be construed as limiting thereof.

EXAMPLES Rational Synthesis of β-Substituted Chlorin Building Blocks

In these examples the synthesis of β-substituted chlorin building blocksis presented. Two new Eastern halves have been constructed in which eachbears one β substituent and one (non-flanking) meso substituent, and onenew Western half has been prepared that bears one β substituent. Thesenew precursors have been used in conjunction with the prior Western half(1) to yield three new chlorins each bearing one β and one mesosubstituent. A chlorin bearing one meso substituent and substituents atthe 2 and 12 positions also has been prepared. Such building blocks haveheretofore not been available and in conjunction with themeso-substituted chlorins previously disclosed (synthesis summarized inFIG. 53), should enable a variety of fundamental studies, includinginvestigation of the effects of site of linker connection on electroniccommunication in various chlorin-based architectures.

Results and Discussion

Synthesis of the Eastern Half (EH): The synthesis of the β-substitutedEH begins in the same manner as the prior synthesis of β-substituteddipyrromethanes (Balasubramanian, T.; Lindsey, J. S. Tetrahedron 1999,55, 6771-6784) but employs a number of significant improvements (FIG.54). The iodophenyl substituted pyrrole (3) is readily prepared from4-iodobenzaldehyde, monoethyl malonate, and tosylmethylisocyanide. Theethoxycarbonyl group was removed by treatment with NaOH in ethyleneglycol at 160° C. to give the 3-(4-iodophenyl)pyrrole (4) in 91% yieldas pale brown crystals. It is noteworthy that this single-stepdecarboxylation is superior to the two-step transformation on similarpyrrole compounds (Pavri, N. R.; Trudell, M. L. J. Org. Chem. 1997, 62,2649-2651). Vilsmeier-Haack formylation of 4 yielded a mixture of tworegioisomers (˜6:1 ratio) which were readily distinguished by ¹H NMRspectroscopy (See Experimental Section). The major isomer was thedesired compound (5) and was obtained in pure form by recrystallizationin 62% yield. Protection of the pyrrolic nitrogen with the BOC group(Tietze, L. F.; Kettschau, G.; Heitmann, K. Synthesis 1996, 851-857)gave pyrrole 6 in quantitative yield. Reduction to alcohol 7 wasachieved by treatment with LiBH₄ at low temperature (longer reactiontime or higher temperature led to the over-reduced and deprotectedcompound 2-methyl-3-(4-iodophenyl)pyrrole). Treatment of 7 with excesspyrrole under acidic conditions furnished the β-substituted,mono-protected dipyrromethane 8 in 68% yield. Excess pyrrole isnecessary to minimize the formation of the tripyrromethane, whileprotection of the pyrrolic nitrogen is necessary to facilitate thereaction, avoid self condensation and allow the subsequent selectivemonoacylation. This methodology afforded the β-substituteddipyrromethane as a single regioisomer, in contrast to earliermethodology which gave a mixture of two regioisomers (Balasubramanian,T.; Lindsey, J. S. Tetrahedron 1999, 55, 6771-6784).

Methods for acylation of 5-substituted dipyrromethanes have beendeveloped that involve formation of the pyrrolic Grignard reagentfollowed by treatment with an acid chloride (Lee, C.-H.; Li, F.;Iwamoto, K.; Dadok, J.; Bothner-By, A. A.; Lindsey, J. S. Tetrahedron1995, 51, 11645-11672). In this case, the N-protected dipyrromethane wasretained for selective monoacylation of the α-position in theunprotected pyrrole unit. Treatment of 8 with 2.5 equivalents of EtMgBrin THF followed by p-toluoyl chloride afforded the monoacylateddipyrromethane 9 in 66% yield (FIG. 54). However, similar reaction intoluene led to a mixture of the mono-acylated product, deprotectedcompound and some unidentified impurities. A control experimentinvolving treatment of 8 with a slight excess of EtMgBr at 0° C. in THFfor 1 h and the usual workup afforded the starting material inquantitative yield, thus revealing that the BOC group is stable to theacylation conditions. Removal of the BOC group under standard conditions(Hasan, I. et al., J. Org. Chem. 1981, 46, 157-164) gave 10.Electrophilic bromination of 10 with NBS (1 equiv) in THF at −78° C.following earlier methods (excess NBS led to a considerable amount of adibromo compound) afforded 11.

A second β-substituted dipyrromethane was prepared by Sonogashiracoupling (Sonogashira, K. et al., Tetrahedron Lett. 1975, 4467-4470) ofiodophenyl-substituted 10 with trimethylsilylacetylene. In this mannerthe trimethylsilylethynyl dipyrromethane 12 was obtained in quantitativeyield (FIG. 55). Reaction of 12 with NBS at −78° C. furnished thecorresponding bromodipyrromethane 13 in 91% yield.

The preparation of a dipyrromethane bearing a substituent at a differentβ site using the same BOC protected dipyrromethane 8 was also soughtreversing the order of acylation and deprotection that led to 10. Thus,deprotection of 8 with NaOMe/MeOH afforded the β-substituteddipyrromethane 14 (FIG. 55). A procedure was recently devised for theselective mono-acylation of meso-substituted dipyrromethanes usingEtMgBr and an S-pyridyl substituted benzothioate (Rao, P. D.;Dhanalekshmi, S.; Littler, B. J.; Lindsey, J. S. J. Org. Chem.submitted). Application of this monoacylation method to 14 resulted in amixture of two regioisomers (10, 16). Attempts to obtain 16 as the majorproduct by varying the experimental conditions were unsuccessful.Separation of the two regioisomers was difficult and required extensiveflash column chromatography. The minor isomer 16 was obtained in 25%yield. Treatment of 16 with 1 equivalent of NBS in THF at −78° C.yielded 17 in 87% yield as a yellow solid. All β-substituted1-bromodipyrromethanes (11, 13, 17) are somewhat unstable but remainintact for a few weeks upon storage at 0° C. under argon.

Synthesis of a β-substituted Western Half. The synthesis of a Westernhalf lacking any β-substituents except for the geminal dimethyl group(1) was previously developed (Strachan, J. P. et al., J. Org. Chem.2000, 65, 3160-3172). Pyrrole-carboxaldehyde 5, available in multi-gramquantities, provided a convenient starting point for the synthesis of anew Western half bearing a synthetic handle at a β position. Aβ-substituted Western half in conjunction with the β-substituted Easternhalf would enable the synthesis of chlorin building blocks bearing two βsubstituents positioned at opposite sides of the macrocycle. Applicationof the reaction conditions used to obtain 2-(2-nitrovinyl)pyrrole from2-formylpyrrole (Strachan, J. P.; O'Shea, D. F.; Balasubramanian, T.;Lindsey, J. S. J. Org. Chem. 2000, 65, 3160-3172) to the reaction of 5resulted largely in recovery of starting material. After a limitedstudy, it was found that treatment of 5 with KOAc and a slight excess ofmethylamine-hydrochloride in nitromethane (instead of methanol) assolvent at room temperature for 2 h (instead of 16 h) yielded thedesired aldol-condensation product 18 in 89% yield (FIG. 56). It isnoteworthy that a longer reaction time led to the formation of theMichael addition product of nitromethane at the nitrovinyl group in 18,forming 2-(1,3-dinitro-2-propyl)-3-(4-iodophenyl)pyrrole in ˜30% yield.NaBH₄ reduction of 18 gave 19, which underwent Michael addition withmesityl oxide in the presence of CsF at 80° C. to give thenitro-hexanone product 20, the precursor to the β-substituted Westernhalf. Although the Michael addition was fast compared to that formingthe β-unsubstituted counterpart (precursor to 1), the yield was slightlylower (42% vs. 65%). Treatment of 20 with NaOMe followed by a bufferedTiCl₃ solution yielded the β-substituted Western half 21 in 45-50% yieldas a light green solid. The yield and stability of the β-substituted WHis greater than that of the unsubstituted analog (21 has mp=141-142° C.;1 is an oil).

Chlorin Formation. Prior synthesis of chlorins involved (1) formation ofthe bromodipyrromethane-monocarbinol (2-OH, EH) by reduction of thecarbonyl group in the EH precursor, (2) acid-catalyzed condensation ofthe EH and WH (1) to obtain the dihydrobilene-α, and (3) oxidativemetal-mediated cyclization to give the chlorin (Strachan, J. P.; O'Shea,D. F.; Balasubramanian, T.; Lindsey, J. S. J. Org. Chem. 2000, 65,3160-3172). All the three steps are done in succession on the same day.This same procedure was employed herein except that the workupconditions are different due to the labile nature of the β-substitutedEH precursors (11, 13, 17) and corresponding β-substituted Easternhalves. In a typical reaction, 11 was treated with NaBH₄ in THF/MeOH(4:1) at room temperature under argon. Upon the disappearance ofstarting material (TLC analysis), the reaction mixture was worked up andthe carbinol 11-OH was treated with 1.2 equivalents of WH 1 at roomtemperature in CH₃CN containing TFA. After 25-30 minutes the resultingdihydrobilene-α was obtained by quenching the reaction mixture withaqueous NaHCO₃ and workup in CH₂Cl₂. Anhydrous toluene and 15 molarequivalents each of AgIO₃, Zn(OAc)₂ and piperidine were added, and themixture was heated at 80° C. for ˜2.5 h. The reaction mixture slowlychanged from red to green, indicating the formation of chlorin.Filtration of the reaction mixture through a pad of silica followed bycolumn chromatography afforded the chlorin Zn-22 in >90% purity.Precipitation with CH₂Cl₂/hexanes furnished pure chlorin (Zn-22) in 18%yield (FIG. 57). Similar treatment of Eastern half 13-OH and 1 gave thezinc chlorin Zn-23 in 22% yield. The Eastern half (17) bearing asubstituent at position 8 reacted similarly with 1 affording zincchlorin Zn-24.

The chlorins Zn22-24 each bear one β substituent. In order to prepare achlorin bearing two β substituents, 13-OH and Western half 21 werereacted to give zinc chlorin Zn-25 in 24% yield (FIG. 58). This chlorinhas an iodophenyl group and an ethynylphenyl group at β positions onopposite sides of the macrocycle. Porphyrins bearing iodophenyl andethynylphenyl groups in a trans orientation have been employed in thestepwise synthesis of linear multi-porphyrin arrays (Wagner, R. W.;Lindsey, J. S. J. Am. Chem. Soc. 1994, 116, 9759-9760; Wagner, R. W.;Ciringh, Y.; Clausen, P. C.; Lindsey, J. S. Chem. Mater. 1999, 11,2974-2983; Lindsey, J. S. et al., Tetrahedron 1994, 50, 8941-8968).Analogous linear multi-chlorin arrays should be attainable with Zn-25.

In each of these chlorin-forming reactions only one chlorin product wasobtained, indicating the absence of scrambling during the course of thereaction. This methodology is quite general and the yields of 18-24%obtained with the three β-substituted Eastern halves (11-OH, 13-OH,17-OH) and the β-substituted Western half (21) are noticeably superiorto the ˜10% obtained with the meso-substituted Eastern halves (2-OH) andWestern half (1).

The Zn-chlorins were demetalated to give the corresponding free basechlorins by treatment with TFA in CH₂Cl₂. In most cases the crudeproduct was pure enough for analysis while in other cases the free basechlorin was further purified by a short silica column.

Spectral Properties of the Chlorins. ¹H NMR Spectra. The NMR spectralinformation available for chlorins has been obtained largely fromnaturally occurring chlorins, which bear alkyl groups at most of the βpositions. The ¹H NMR spectra of β-substituted free base chlorins(22-25) and Zn chlorins (Zn-22-Zn-25) are readily assignable and confirmthe expected substitution patterns. In 22, the two NH protons appear asbroad peaks at δ −2.15 and −1.85 ppm, and a downfield signal appears forone of the meso substituted protons (assigned to C-10) at δ 9.84 ppm.The reduced ring exhibits a singlet at δ2.07 ppm (geminal dimethylgroups) and another singlet at δ 4.64 ppm (ring CH₂), as also observedin the meso-substituted chlorins. Other characteristic features includean AB quartet at δ 8.85 ppm (β-pyrrole protons of ring A), two doubletsat δ 8.64 and 8.90 ppm (β-pyrrole protons of ring B), and singlets at δ8.91 (for 2H) and 8.99 ppm (two meso protons at C-15 and C-20, and oneβ-pyrrole proton of ring C). The significant changes for theβ-substituted Zn-22 are the absence of signals corresponding to NHprotons, and slight upfield shifts of the geminal dimethyl group (δ 2.01ppm), ring methylene protons (δ 4.48 ppm) and all of the meso andβ-pyrrole protons. Similar trends were observed for free base chlorin 23and zinc chlorin Zn-23.

The ¹H NMR spectrum of chlorin 24 is slightly different due to thedifference in the substitution pattern at the perimeter of the molecule.Characteristic features in addition to the different chemical shifts ofthe two NH protons include the singlet at δ 8.64 ppm (β-pyrrole protonof ring B) and the downfield signal at δ 9.17 ppm as a doublet (one ofthe β-pyrrole protons of ring C). The ¹H NMR spectrum of chlorin 25 ismore simple. The β-pyrrole protons of ring B appear as two doublets at δ8.62 and 8.88, and the AB quartet corresponding to the β-pyrrole protonsof ring A in chlorins 22-24 is absent. The remaining meso protons andβ-pyrrole protons resonate as five singlets. Zn-25 showed a similarpattern except for the slight upfield shift of the peaks due to the mesoand β protons.

A distinctive feature of this set of chlorins is that the β-pyrroleprotons of ring B appear slightly upfield compared to the other pyrroleprotons. This indicates that the β-pyrrole double bond of ring B doesnot participate as fully in the 18ρ electron ring current of the chlorinmacrocycle.

Absorption Spectra. Each of the free base chlorins (22-25) exhibits anintense Soret band and a characteristic strong Q_(y) band. The Soretband in each case exhibited a short-wavelength shoulder of significantintensity, resulting in a fwhm ranging from 32-35 nm for 22-25. Asimilar spectral feature was observed for the previous set ofmeso-substituted free base chlorins that were examined. The Soret bandred-shifted slightly as the substituent was moved from position 8 (24)to 12 (22, 23) to 2 and 12 (25). Significant differences in Q_(y)absorption maximum and absorption intensity occurred depending on thesite of substitution of the chlorin. The Q_(y) absorption maximum rangedfrom 637 to 655 nm, and paralleled the redshift of the Soret band. Inaddition, a hyperchromic effect of the Q_(y) band was observedaccompanying the bathochromic shift. Although the accurate determinationof molar absorption coefficients can be difficult especially withhandling small samples, the ratio of the Q_(y) and Soret bands providesa relative measure of the changing band intensities. The Soret/Q_(y)band ratio decreases from 4.3 (24) to 2.5 (25). These data are listed inTable 1. It is noteworthy that the chlorins with an iodophenyl orethynylphenyl group at the 12 position exhibited nearly identicalabsorption spectra. For comparison, the meso-substituted free basechlorins exhibited absorption maxima at 411-414 nm and 640-644 nm.

Each of the zinc chlorins (Zn-22- Zn25) exhibits an intense Soret bandand a characteristic strong Q_(y) band. The Soret band in each case wassharp (fwhm 18-21 nm) with only a very weak short-wavelength shoulder.The Q_(y) band underwent a redshift from 606 nm to 628 nm as thesubstituent location was changed from 8 (Zn-24) to 12 (Zn-22, Zn-23) to2 and 12 (Zn-25). A concomitant increase in intensity of the Q_(y) bandalso was observed. These results are listed in Table 1. In all of thechlorins examined, a redshift in the Soret band was accompanied by amore pronounced redshift in the Q_(y) band. The only discrepancy in thistrend occurred in comparing Zn-24 and Zn-22 (or Zn-23). The former hasthe shortest wavelength Q_(y) band (606 nm) but a Soret band at 415 nm,compared with 615 nm and 411 nm for that of the latter. For comparison,the meso-substituted zinc chlorins exhibited absorption maxima at 412 nmand 608 nm.

Fluorescence Spectra and Yields. Similar to the meso-substitutedchlorins, the free base chlorins 22-24 exhibit a characteristic sharpfluorescence band at 640 nm and a weaker emission in the region 660- 720nm. The latter exhibited two discernible maxima at approximately 680 and710 nm. The emission spectrum of free base chlorin 25 was shifted to 660nm and 726 nm. The Zn chlorins Zn-22 and Zn-23 each exhibit a sharpfluorescence band at around 620 nm and a weak band at 676 nm, whereasthe emission of Zn-24 appears at 609 and 661 nm. The emission spectrumof Zn-25 is more red shifted as observed in free base 25 (635 and 691nm). The fluorescence quantum yields were determined for those chlorinslacking iodophenyl substituents (which exhibit decreased yields due tothe heavy atom effect). The fluorescence quantum yield of free basechlorin 23 was 0.25, while that of Zn-23 was 0.11. These values are inline with those of other naturally occurring or synthetic chlorins.

Conclusions. The synthesis of chlorins described herein provides thefollowing features: (1) control over the location of the reduced ring,(2) locking in of the chlorin hydrogenation level through use of ageminal dimethyl group, (3) location of synthetic handles at designatedsites at the perimeter of the macrocycle, and (4) a single chlorinproduct thereby facilitating purification. The ability to incorporatesubstituents at distinct locations (2, 5, 8, 10, or 12) about thechlorin perimeter opens a number of opportunities. With differentsubstitution patterns, the Q_(y) absorption band can be tuned over therange 637-655 nm for free base chlorins and 606-628 nm for zincchlorins, enabling greater spectral coverage. The chlorin bearingsynthetic handles at the 2 and 12 positions (25) should enable theincorporation of chlorin building blocks into linear architectures. Theavailability of a family of synthetic chlorins bearing diversesubstituents at defined locations should facilitate the systematic studyof substituent effects and broaden the scope of chlorin containing modelsystems.

Experimental Section

General. ¹H and ¹³C NMR spectra (300 MHz) were obtained in CDCl₃ unlessnoted otherwise. Absorption spectra (Cary 3, 0.25 nm data intervals) andfluorescence spectra (Spex FluoroMax, 1 nm data intervals) werecollected routinely. Chlorins were analyzed in neat form by laserdesorption mass spectrometry (LD-MS) in the absence of a matrix (Fenyo,D. et al., J. Porphyrins Phthalocyanines 1997, 1, 93-99; Srinivasan, N.et al., J. Porphyrins Phthalocyanines 1999, 3, 283-291). Pyrrole wasdistilled at atmospheric pressure from CaH₂. Melting points areuncorrected. p-Iodobenzaldehyde was obtained from Karl Industries. Allother reagents and starting materials were obtained from Aldrich.Spectral parameters including molar absorption coefficients andfluorescence quantum yields (Φ_(f)) were determined as previouslydescribed (Strachan, J. P. et al., J. Org. Chem. 2000, 65, 3160-3172).

Chromatography. Preparative chromatography was performed using silica(Baker) or alumina (Fisher A540, 80-200 mesh) and eluting solvents basedon hexanes admixed with ethyl acetate or CH₂Cl₂.

Solvents. THF was distilled from sodium benzophenone ketyl as required.H₃CN (Fisher certified A.C.S.) was distilled from CaH₂ and stored overpowdered molecular sieves. Nitromethane was stored over CaCl₂. CH₂Cl₂was distilled from CaH₂. Dry methanol was prepared as follows. Magnesiumturnings (5 g) and iodine (0.5 g) with 75 mL of methanol were warmeduntil the iodine disappeared and all the magnesium was converted to themethoxide. Up to 1 L of methanol was added and heated at reflux for aminimum of 2 h before collecting. Other solvents were used as received.

Compounds 1 (Strachan, J. P. et al., J. Org. Chem. 2000, 65, 3160-3172)and 3 (Balasubramanian, T.; Lindsey, J. S. Tetrahedron 1999, 55,6771-6784) were prepared according to literature procedures.

3-(4-Iodophenyl)pyrrole (4). Following earlier procedures(Balasubramanian, T.; Lindsey, J. S. Tetrahedron 1999, 55, 6771-6784), amixture of 3-ethoxycarbonyl-4-(4-iodophenyl)pyrrole (7.20 g, 21.1 mmol)and ethylene glycol (55 mL) in a 100-mL Claisen flask was flushed withargon for 10 min and powdered NaOH (2.2 g, 55 mmol) was then added. Theflask was placed in an oil bath at 120° C. and the oil bath temperaturewas raised to 160° C. After 2.5 h, the flask was cooled to roomtemperature and treated with 10% aq NaCl (100 mL). The aqueous layer wasextracted with CH₂Cl₂, the organic layers were collected, washed with10% aq NaCl, dried (Na₂SO₄), concentrated, and recrystallized in ethanolaffording light brown crystals (5.18 g, 91%). mp 164-165° C.; ¹H NMR δ6.51 (m, 1H), 6.83 (m, 1H), 7.08 (s, 1H), 7.27 (d, J=8.7 Hz, 2H), 7.63(d, J=8.7 Hz, 2H); ¹³C NMR δ 89.9,106.3, 114.7, 119.1, 123.8, 127.0,135.2, 137.5; EI-MS obsd 268.9702, calcd 268.9702. Anal. Calcd forC₁₀H₈IN: C, 44.6; H, 3.0; N, 5.2. Found: C, 44.7; H, 3.0; N, 5.1. Thesynthesis starting from 4-iodobenzaldehyde (35 g), monoethyl malonate,and TosMIC has been performed with linear scale up of the establishedprocedures, affording 21.5 g of 4.

2-Formyl-3-(4-iodophenyl)pyrrole (5). A solution of 4 (5.15 g, 19.1mmol) in DMF (6.1 mL) and CH₂Cl₂ (140 mL) under argon was cooled to 0°C. and then POCl₃ (2.11 mL, 22.6 mmol) was added dropwise. After 1 h,the flask was warmed to room temperature and stirred overnight. Thereaction was quenched at 0° C. with 2.5 M NaOH (100 mL). The mixture waspoured into water (500 mL), extracted with CH₂Cl₂, and the combinedorganic layers were washed with water, brine, dried (Na₂SO₄), and thesolvent was removed in vacuo. ¹H NMR spectroscopy showed tworegioisomers in a ˜6:1 ratio. The minor isomer exhibited signals at δ7.21 and 7.39 ppm, compared with signals at δ 6.42 and 7.14 for themajor isomer. The most downfield signal (7.39 ppm) is assigned to theproton adjacent to a formyl group, which occurs in the 2-formyl-4-arylsubstituted pyrrole. Recrystallization from ethyl acetate afforded anorange solid corresponding to the major aldehyde (2.25 g). The motherliquor was concentrated and purified by flash column chromatography[silica, hexanes/ethyl acetate (3:1)]. The first fraction correspondedto the major aldehyde (1.25 g). The total yield of the title compoundwas 3.50 g (62%): mp 153-154° C.; ¹H NMR δ 6.42 (m, 1H), 7.14 (m, 1H),7.22 (m, 2H), 7.76 (m 2H), 9.59 (s, 1H), 10.72 (br, 1H); ¹³C NMR δ 93.5,104.3, 111.4, 125.8, 128.6, 130.8, 133.1, 137.8, 179.4; FAB-MS obsd296.9663, calcd 296.9651; Anal. Calcd for C₁₀H₈INO: C, 44.5; H, 2.7; N,4.7. Found: C, 44.4; H, 2.7; N, 4.6.

N-tert-Butoxycarbonyl-2-formyl-3-(4-iodophenyl)pyrrole (6). Following astandard procedure (Tietze, L. F.; Kettschau, G.; Heitmann, K. Synthesis1996, 851-857), sample of NaH (70 mg, 1.75 mmol, 60% dispersion inmineral oil) in a round-bottomed flask under argon was washed twice withanhydrous pentane (˜5 mL). Anhydrous THF (14 mL) was added followed by 5(400 mg, 1.35 mmol). After stirring for 30 min at room temperature,(BOC)₂O (325 mg, 1.5 mmol) was added and stirring was continued foranother 2 h. The reaction was quenched with 50% satd. aq NH₄Cl (50 mL).The mixture was extracted with ether, and the combined organic layerswere washed with brine, dried (Na₂SO₄), and filtered [silica,hexanes/ethyl acetate (4:1)] to yield a viscous oil (535 mg,quantitative). ¹H NMR δ 1.64 (s, 9H), 6.33 (d, J=3.0 Hz, 1H), 7.30 (d,J=8.1 Hz, 2H), 7.46 (d, J=3.0 Hz, 1H), 7.72 (d, J=8.1 Hz, 2H), 10.22 (s,1H); ¹³C NMR δ 27.7, 85.8, 94.2, 113.2, 126.7, 128.5, 131.3, 132.8,137.0, 137.4, 148.3, 181.6; FAB-MS obsd 397.0176, calcd 397.0175(C₁₆H₁₆INO₃).

N-tert-Butoxycarbonyl-2-hydroxymethyl-3-(4-iodophenyl)pyrrole (7). Asolution of 6 (400 mg, 1.0 mmol) in anhydrous THF (12 mL) under argonwas cooled to −20 to −25° C. and LiBH4 (55 mg, 2.5 mmol) was added inportions. The reaction was monitored by TLC (silica, hexanes/ethylacetate (4:1)), and when no starting material was detected (20-25 min),the reaction was quenched with cold water (30 mL). The aqueous layer wasextracted with CH₂Cl₂ and the organic layer was dried (Na₂SO₄),concentrated, and purified by flash column chromatography [silica,hexanes/ethyl acetate containing 1% Et₃N (3:1)] yielding a gum (330 mg,82%). ¹H NMR δ 1.62 (s, 9H), 3.61 (br, 1H), 4.66 (d, J=7.2 Hz, 2H), 6.25(d, J=3.6 Hz, 1H), 7.18 (d, J=8.1 Hz, 2H), 7.22 (d, J=3.6 Hz, 1H), 7.71(d, J=8.1 Hz, 2H); ¹³C NMR δ 27.8, 55.3, 84.7, 92.4, 111.2, 121.3,127.9, 130.0, 130.4, 134.1, 137.5, 149.8; FAB-MS obsd 399.0336, calcd399.0331 (C₁₆H₁₈INO₃).

3-(4-Iodophenyl)-10-N-(tert-butoxycarbonyl)dipyrromethane (8). Asolution of 7 (1.2 g, 3.0 mmol) and pyrrole (3.36 mL, 48 mmol) in1,4-dioxane (36 mL) at room temperature was treated with 10% aq HCl (6.0mL). The reaction was monitored by TLC [silica, hexanes/ethyl acetate(4:1)]. After 4 h, satd aq NaHCO₃ (50 mL) and water (50 mL) were addedand the mixture was extracted with CH₂Cl₂. The combined organic layerswere washed with water, brine, dried (Na₂SO₄), concentrated, andpurified by flash chromatography [silica, hexanes/ethyl acetate (4:1)].A non-polar product was isolated in minor amount (uncharacterized). Thedesired product was obtained as a pale brown solid (920 mg, 68% yield):mp 128-129 ° C.; ¹H NMR δ 1.57 (s, 9H), 4.18 (s, 2H), 5.87 (br, 1H),6.10 (m, 1H), 6.22 (d, J=3.0 Hz, 1H), 6.64 (m, 1H), 7.16 (d, J=8.0 Hz,2H), 7.24 (d, J=3.6 Hz, 1H), 7.71 (d, J=8.0 Hz, 2H), 8.78 (br, 1H); ¹³CNMR δ 24.6, 27.8, 84.3, 92.1, 105.8, 107.9, 111.6, 116.3, 121.0, 126.8,128.5, 130.4, 130.8, 135.0, 137.4, 150.0; FAB-MS obsd 448.0659, calcd448.0648; Anal. Calcd for C₂₀H₂₁IN₂O₂: C, 53.6; H, 4.7; N, 6.3. Found:C, 54.1; H, 4.9; N, 5.9.

3-(4-lodophenyl)-9-(4-methylbenzoyl)-10-N-(tert-butoxycarbonyl)dipyrromethane(9). A solution of 8 (448 mg, 1.0 mmol) in anhydrous THF (15 mL) underargon at 0° C. was treated slowly with EtMgBr (1 M in THF, 2.5 mL, 2.5mmol). The mixture was stirred for 30 minutes at 0° C., then, p-toluoylchloride (200 μL, 1.5 mmol) was added slowly and stirring was continuedfor 1 h at 0° C. The reaction was quenched with satd. aq NH₄Cl andextracted with CH₂Cl₂. The combined organic layers were washed withwater, brine, dried (Na₂SO₄), concentrated, and the product was purifiedby flash column chromatography [silica, hexanes/ethyl acetate (4:1)].The product was obtained as a pale white solid (375 mg, 66%): mp120-121° C.; (due to possible rotamers the ¹H NMR and ¹³C NMR are notvery clean) ¹H NMR δ 1.56 (s, 9H), 2.42 (s, 3H), 4.29 (s, 2H), 5.95 (m,1H), 6.26 (m, 1H), 6.76 (m, 1H), 7.09 (m, 2H), 7.16 (m, 1H), 7.25 (m,2H), 7.31 (m, 1H), 7.71 (d, 8.7 Hz, 2H), 7.77 (d, J=8.1 Hz, 2H), 9.95(br, 1H); ¹³C NMR δ 25.2, 27.8, 31.7, 84.8, 92.3, 109.3, 111.5, 119.6,121.5, 125.8, 126.3, 128.9, 129.0, 130.2, 130.6, 134.7, 135.8, 137.5,138.6, 142.0, 149.7, 183.8; Anal. Calcd for C₂₈H₂₇IN₂O₃: C, 59.4; H,4.8; N, 5.0. Found: C, 59.4; H, 4.6; N, 5.1.

3-(4-Iodophenyl)-9-(4-methylbenzoyl)dipyrromethane (10). Following astandard method for the deprotection of BOC-protected pyrroles (Hasan,I. et al., J. Org. Chem. 1981, 46, 157-164), a solution of 9 (328 mg,0.58 mmol) in anhydrous THF (4 mL) under argon at room temperature wastreated with methanolic NaOMe (0.7 mL, prepared by dissolving 200 mg ofNaOMe in 1.0 mL of MeOH). After 20-25 min, the reaction was quenchedwith a mixture of hexanes and water (20 mL, 1:1) and extracted withethyl acetate. The combined organic layers were washed with water,brine, dried (Na₂SO₄), and purified by flash column chromatography[silica, hexanes/ethyl acetate (3:1)] to yield a pale brown solid (216mg, 80%): mp 185-186° C.; ¹H NMR δ 2.43 (s, 3H), 4.17 (s, 2H), 6.15 (m,1H), 6.56 (m, 1H), 6.85 (m, 1H), 7.17 (m, 2H), 7.28 (m, 2H), 7.69 (m,2H), 7.77 (d, J=7.8 Hz, 2H), 9.43 (br, 1H), 10.88 (br, 1H); ¹³C NMR δ21.6, 25.2, 90.6, 108.6, 110.3, 117.4, 121.1, 122.3, 123.9, 129.0,129.1, 130.0, 130.7, 135.5, 136.2, 137.4, 139.4, 142.6, 185.2; FAB-MSobsd 466.0561, calcd 466.0542; Anal. Calcd for C₂₃H₁₉IN₂O: C, 59.2; H,4.1; N, 6.0. Found: C, 59.3; H, 4.2; N, 5.9.

1-Bromo-3-(4-iodophenyl)-9-(4-methylbenzoyl)dipyrromethane (11).Following earlier procedures (Strachan, J. P. et al., J. Org. Chem.2000, 65, 3160-3172), 10 (120 mg, 0.26 mmol) was dissolved in anhydrousTHF (6 mL) and cooled to −78° C. under argon. Recrystallized NBS (46 mg,0.26 mmol) was added and the reaction mixture was stirred for 1 h (−78°C.) and then quenched with a mixture of hexanes and water (20 mL, 1:1)and allowed to warm to 0° C. The aqueous portion was extracted withreagent-grade ether and the combined organic layers were dried overK₂CO₃. The solvent was evaporated under vacuum at room temperature.Purification by flash column chromatography [silica, hexanes/ether(2:1)] afforded a yellow solid (120 mg, 85%). The bromodipyrromethane isunstable but can be stored for several weeks at 0° C. mp 160° C.(decomp); ¹H NMR δ 2.44 (s, 3H), 4.09 (s, 2H), 6.12 (d, J=3.0 Hz, 1H),6.16 (m, 1H), 6.89 (m, 1H), 7.14 (d, J=7.8 Hz, 2H), 7.30 (d, J=7.8 Hz,2H), 7.71 (d, J=8.1 Hz, 2H), 7.80 (d, J=8.1 Hz, 2H), 10,33 (br, 1H),11.59 (br, 1H); ¹³C NMR δ 21.6, 24.9, 91.1, 97.9, 110.2, 110.5, 122.8,123.5, 125.4, 129.2, 130.2, 130.0, 130.8, 135.2, 135.4, 137.5, 139.9,142.8, 186.1; FAB-MS obsd 543.9642, calcd 543.9647; Anal. Calcd forC₂₃H₁₈BrIN₂O: C, 50.7; H, 3.3; N, 5.1. Found: C, 51.3; H, 3.5; N, 5.2.

3-[4-(Trimethylsilylethynyl)phenyl]-9-(4-methylbenzoyl)dipyrromethane(12). Samples of 10 (279 mg, 0.599 mmol), Pd₂(dba)₃ (42 mg, 0.046 mmol),Ph₃As (113 mg, 0.369 mmol), and CuI (9 mg, 0.047 mmol) were added to a25-mL Schlenk flask. The flask was evacuated and purged with argon forthree times. Then deaerated anhydrous THF/Et₃N (6 mL, 1:1) was added andfollowed by trimethylsilylacetylene (127 μL, 0.90 mmol). The flask wassealed, immersed in an oil bath (37° C.), and the mixture was stirredovernight. Then CH₂Cl₂ (20 mL) was added and the mixture was filteredthrough a pad of Celite, washed several times with CH₂Cl₂, concentrated,and the residue was purified by flash column chromatography [silica,hexanes/ethyl acetate (3:1)] to afford a yellow solid (262 mg,quantitative): mp 126-127° C.; ¹H NMR δ 0.26 (s, 9H), 2.43 (s, 3H), 4.19(s, 2H), 6.16 (m, 1H), 6.28 (m, 1H), 6.55 (m, 1H), 6.85 (m, 1H), 7.28(d, J=8.7 Hz, 2H), 7.38 (d, J=8.1 Hz, 2H), 7.49 (d, J=8.7 Hz, 2H), 7.77(d, J=8.1 Hz, 2H), 9.51 (br, 1H), 10.96 (br, 1H); ¹³C NMR δ 0.0, 21.5,25.3, 105.4, 108.6, 110.3, 117.4, 119.9, 121.5, 122.3, 124.1, 127.6,129.0, 129.1, 130.7, 132.0, 135.5, 137.0, 139.5, 142.6, 185.2; FAB-MSobsd 436.1972, calcd 436.1971; Anal. Calcd for C₂₈H₂₈N₂OSi: C, 77.0; H,6.5; N, 6.4. Found: C, 76.3; H, 6.3; N, 6.3.

1-Bromo-3-[4-(trimethylsilylethynyl)phenyl]-9-(4-methylbenzoyl)dipyrromethane(13). Following the procedure for the synthesis of 11, treatment of 12(150 mg, 0.34 mmol) with NBS (60 mg, 0.34 mmol) afforded a pale yellowsolid (160 mg, 91%): mp 140° C. (decomp); ¹H NMR δ 0.26 (s, 9H), 2.44(s, 3H), 4.12 (s, 2H), 6.17 (m, 2H), 6.89 (m, 1H), 7.31 (m, 4H), 7.50(d, J=9.0 Hz, 2H), 7.80 (d, J=8.1 Hz, 2H), 10.16 (br, 1H), 11.42 (br,1H); ¹³C NMR δ 0.0, 21.5, 25.0, 94.1, 97.9, 105.2, 110.3, 110.5, 120.4,123.3, 125.5, 127.7, 129.2, 130.7, 132.1, 135.4, 135.9, 139.7, 142.8,185.9; FAB-MS obsd 514.1079, calcd 514.1076; Anal. Calcd forC₂₈H₂₇BrN₂OSi: C, 65.2; H, 5.3; N, 5.4. Found: C, 65.1; H, 5.2; N, 5.3.

3-(4-Iodophenyl)dipyrromethane (14). Following the deprotectionprocedure used to prepare 10, a sample of 8 (225 mg, 0.50 mmol) inanhydrous THF (4 mL) under argon at room temperature was treated withmethanolic NaOMe (0.6 mL, prepared by dissolving 200 mg of NaOMe in 1.0mL of MeOH). After 15 min, the reaction was quenched with mixture ofhexanes and water (14 mL, 1:1), extracted with ethyl acetate, and thecombined organic layers were washed with water, brine, then dried overNa₂SO₄. The residue was passed through a filtration column to yield alight brown solid (160 mg, 92%). Analytical data are in accord with theliterature (Balasubramanian, T.; Lindsey, J. S. Tetrahedron 1999, 55,6771-6784).

3-(4-Iodophenyl)-1-(4-methylbenzoyl)dipyrromethane (16). Following ageneral monoacylation procedure for unprotected dipyrromethanes (Rao, P.D. et al., J. Org. Chem. 2000, 65, 1084-1092.), EtMgBr (1 M solution inTHF, 2.2 mL, 2.2 mmol) was added to a solution of 14 (385 mg, 1.1 mmol)in anhydrous THF (14 mL). After stirring for 10 min, the flask wascooled to −78° C. and a solution of pyridyl thioester 15 (255 mg, 1.1mmol) in anhydrous THF (3 mL) was added slowly. After a few min thecooling bath was removed, stirring was continued for 1 h, then themixture was quenched with satd aq NH₄Cl, water, and then extracted withCH₂Cl₂. The combined organic layers were washed with water, brine, dried(Na₂SO₄), and concentrated. The two regioisomers formed were purified bytwo successive flash columns [silica, hexanes/ethyl acetate (3:1)],affording the minor isomer 16 (130 mg, 25%) and the major isomer 10 (270mg, 53%). Data for 16: mp 190° C. (decomp); ¹H NMR δ 2.43 (s, 3H), 4.15(s, 2H), 6.05 (m, 1H), 6.13 (m, 1H), 6.58 (m, 1H), 6.94 (m, 1H), 7.19(d, J=8.7 Hz, 2H), 7.28 (d, J=8.1 Hz, 2H), 7.73 (d, J=8.7 Hz, 2H), 7.78(d, J=8.1 Hz, 2H), 9.17 (br, 1H), 10.83 (br, 1H); ³C NMR δ 21.6, 25.2,91.8, 106.8, 108.3, 117.8, 121.1, 124.3, 127.2, 129.1, 129.2, 129.7,130.2, 134.6, 135.4, 136.3, 137.6, 142.9, 185.4; FAB-MS obsd 466.0573,calcd 466.0542; Anal. Calcd for C₂₃H₁₉IN₂O: C, 59.2; H, 4.1; N, 6.0.Found: C, 59.1; H, 4.2; N, 5.8.

9-Bromo-3-(4-iodophenyl)-1-(4-methylbenzoyl)dipyrromethane (17).Following the procedure for the synthesis of 11, treatment of 16 (186mg, 0.400 mmol) with NBS (72 mg, 0.405mmol) gave a pale yellow solid(189 mg, 87%): mp 140° C. (decomp); ¹H NMR δ 2.43 (s, 3H), 4.08 (s, 2H),5.94 (m, 1H), 6.00 (m, 1H), 6.96 (d, J=2.1 Hz, 1H), 7.18 (d, J=8.7 Hz,2H), 7.29 (d, J=8.1 Hz, 2H), 7.74 (d, J=8.7 Hz, 2H), 7.80 (d, J=8.1 Hz,2H), 9.80 (br, 1H), 11.53 (br, 1H); ¹³C NMR was attempted in CDCl₃ butthe compound decomposed upon lengthy data acquisition. FAB-MS obsd543.9628, calcd 543.9647; Anal. Calcd for C₂₃H₁₉IN₂O: C, 50.7; H, 3.3;N, 5.1. Found: C, 51.2; H, 3.4; N, 5.0.

2-(2-trans-Nitrovinyl)-3-(4-iodophenyl)pyrrole (18). A mixture of 5(1.485 g, 5.00 mmol), KOAc (492 mg, 5.01 mmol),methylamine-hydrochloride (402 mg, 5.95 mmol), and nitromethane (45 mL)under argon was stirred at room temperature. The mixture slowly becameorange and yielded an orange-red precipitate. The reaction was monitoredby TLC and after stirring for 2 h, the TLC showed the appearance of anew component and the disappearance of 5. (A longer reaction time (10 h)led to formation of the Michael addition product,2-(1,3-dinitro-2-propyl)-3-(4-iodophenyl)pyrrole, in ˜30% yield.) Thereaction was quenched with brine, extracted with ethyl acetate, and theorganic layers were dried (Na₂SO₄) and concentrated. The residue wastreated with hot ethyl acetate and filtered, then concentrated anddissolved in hot CH₂Cl₂, followed by precipitation upon adding coldhexanes, affording an orange solid (1.52 g, 89%): mp 217-218° C.(decomp); ¹H NMR (acetone-d₆) δ 6.56 (d, J=2.1 Hz, 1H), 7.32 (d, J=8.2Hz, 2H), 7.35 (m, 1H), 7.81 (d, J=13.5 Hz, 1H), 7.90 (d, J=8.3 Hz, 2H),7.99 (d, J=13.4 Hz, 1H); ¹³C NMR (acetone-d₆) δ 93.4, 112.5, 121.3,127.1, 127.2, 128.4, 131.8, 132.8, 135.4, 138.9; FAB-MS obsd 339.9720,calcd 339.9709. Anal. Calcd for C₁₂H₉IN₂O₂: C, 42.4; H, 2.7; N, 8.2.Found: C, 41.8; H, 2.6; N, 7.9; λ_(abs) (toluene) 395 nm.

2-(2-Nitroethyl)-3-(4-iodophenyl)pyrrole (19). Following the procedurefor a β-unsubstituted pyrrole (Strachan, J. P. et al., J. Org. Chem.2000, 65, 3160-3172), a sample of 18 (1.36 g, 4.00 mmol) was dissolvedin anhydrous THF/MeOH (40 mL, 9:1) under argon at 0° C. NaBH₄ (605 mg,16.00 mmol) was added in portions and stirring was continued for 1 h at0° C. Then the mixture was stirred for 2 h at room temperature. Thereaction mixture was neutralized with acetic acid (pH=7), then water (50mL) was added and the mixture was extracted with ethyl acetate. Thecombined organic layers were washed with water, brine, dried (Na₂SO₄),concentrated, and purified by passage through a short column [silica,hexanes/ethyl acetate (3:1)] to give a pale white solid (1.2 g, 88%): mp88-89° C.; ¹H NMR δ 3.41 (t, J=6.6 Hz, 2H), 4.52 (t, J=6.6 Hz, 2H), 6.26(s, 1H), 6.74 (s, 1H), 7.07 (d, J=8.1 Hz, 2H), 7.69 (d, J=8.1 Hz, 2H),8.33 (br, 1H); ¹³C NMR δ 24.0, 75.0, 91.1, 109.3, 117.8, 122.1, 122.2,129.8, 135.7, 137.7; FAB-MS obsd 341.9877, calcd 341.9865; Anal. Calcdfor C₁₂H₁₁IN₂O₂: C, 42.1; H, 3.2; N, 8.2. Found: C, 42.3; H, 3.3; N,8.1.

1-[3-(4-Iodophenyl)pyrro-2-yl]-2-nitro-3,3-dimethyl-5-hexanone (20).Following the procedure for a β-unsubstituted pyrrole (Strachan, J. P.et al., J. Org. Chem. 2000, 65, 3160-3172), a mixture of 19 (1.03 g, 3.0mmol), CsF (2.28 g, 15.0 mmol), and mesityl oxide (1.72 mL, 15.0 mmol)in anhydrous acetonitrile (22.5 mL) was heated at 80° C. for 2.5 h to 3h, during which the mixture turned from colorless to brown and then darkred. TLC analysis confirmed the absence of starting material. Thesolvent was evaporated under vacuum, the residue was taken up in ethylacetate and filtered through a pad of silica using ethyl acetate aseluant. The solvent was evaporated under vacuum and the product waspurified by a gravity column [alumina, hexanes/ethyl acetate (2:1)]followed by recrystallization from CH₂Cl₂/hexanes to afford browncrystals (550 mg, 42%): mp 124-125° C.; ¹H NNR δ 1.08 (s, 3H), 1.19 (s,3H), 2.11 (s, 3H), 2.37 (d, J=17.4 Hz, 1H), 2.56 (d, J=17.4 Hz, 1H),3.15 (m, 1H), 3.39 (m, 1H), 5.20 (m, 1H), 6.21 (m, 1H), 6.68 (m, 1H),7.10 (m, 2H), 7.70 (m, 2H), 8.22 (br, 1H); ¹³C NMR δ 23.9, 24.2, 24.8,31.6, 36.8, 51.2, 91.1, 94.2, 109.1, 117.8, 122.2, 122.4, 130.1, 135.9,137.5, 206.7; FAB-MS obsd 440.0605, calcd 440.0597; Anal. Calcd forC₁₈H₂₁IN₂O₃: C, 49.1; H, 4.8; N, 6.4. Found: C, 49.1; H 4.7; N, 6.3.

1,3,3-Trimethyl-7-(4-iodophenyl)-2,3-dihydrodipyrrin (21). Following theprocedure for a β-unsubstituted pyrrole (Strachan, J. P. et al., J. Org.Chem. 2000, 65, 3160-3172), a solution of 20 (220 mg, 0.50 mmol) inanhydrous THF (5.0 mL) under argon was treated with NaOMe (135 mg, 2.5mmol) and the mixture was stirred for 1 h at room temperature (firstflask). In a second flask, TiCl₃ (8.6 wt % TiCl₃ in 28 wt % HCl, 3.8 mL,2.5 mmol, 5.0 mol equivalent) and H₂O (20 mL) were combined, NH₄OAc (˜15g) was added to buffer the solution to pH 6.0, and then THF (5 mL) wasadded. The nitronate anion of 20 formed in the first flask wastransferred via a cannula to the buffered TiCl₃ solution in the secondflask. Additional THF (3 mL) was added to the nitronate anion flask andthe supernatant was also transferred to the buffered TiCl₃ solution. Theresulting mixture was stirred at room temperature for 6 h under argon.Then the mixture was extracted with ethyl acetate and the combinedorganic layers were washed with satd aq NaHCO₃, water, brine, and thendried (MgSO₄). The solvent was removed under reduced pressure at roomtemperature. The crude product was passed through a short column[alumina, hexanes/ethyl acetate (2:1)] to afford a light green solid(80-92 mg, 45-50%): mp 140-142° C.; ¹H NMR δ 1.18 (s, 6H), 2.22 (s, 3H),2.52 (s, 2H), 5.89 (s, 1H), 6.26 (m, 1H), 6.85 (m, 1H), 7.19 (m, 2H),7.69 (m, 2H), 11.09 (br, 1H); ¹³C NMR δ 20.7, 29.1, 29.7, 41.2, 53.7,90.3, 102.3, 108.6, 118.5, 122.2, 127.5, 130.4, 136.8, 137.4, 161.9,177.2; FAB-MS obsd 390.0595, calcd 390.0593 (C₁₈H₁₉IN₂); λ_(abs)(toluene) 352 nm.

General procedure for chlorin formation:Zn(II)-17,18-Dihydro-18,18-dimethyl-5-(4-methylphenyl)-12-(4-iodophenyl)porphyrin(Zn-22). Following a general procedure (Strachan, J. P. et al., J. Org.Chem. 2000, 65, 3160-3172), to a solution of 11 (110 mg, 0.20 mmol) in7.5 mL of anhydrous THF/MeOH (4:1) at room temperature was added excessof NaBH₄ (100 mg, 2.6 mmol) in small portions. The reaction wasmonitored by TLC [alumina, hexanes/ethyl acetate (3:1)] and uponcompletion the mixture was quenched with cold water (˜10 mL), thenextracted with distilled CH₂Cl₂ (3×25 mL). The combined organic layerswere washed with brine (50 mL), dried (K₂CO₃) for 2-3 min, andconcentrated in vacuo at room temperature to leave the resultingcarbinol 11-OH in ˜1-2 mL of CH₂Cl₂. The WH 1 (45 mg, 0.24 mmol) wasdissolved in a few mL of anhydrous CH₃CN and combined with 11-OH, thenadditional anhydrous CH₃CN was added to give a total of 22 mL CH₃CN. Thesolution was stirred at room temperature under argon and TFA (20 μL,0.26 mmol) was added. The reaction was monitored by TLC [alumina,hexanes/ethyl acetate (3:1)], which after 25-30 min showed thedisappearance of the EH and the appearance of a bright spot just belowthe WH. The reaction mixture was quenched with 10% aq NaHCO₃ andextracted with distilled CH₂Cl₂ (3×25 mL). The combined organic layerswere washed with water, brine, dried (Na₂SO₄) and the solvent wasremoved in vacuo at room temperature. The residue was dissolved in 14 mLof anhydrous toluene under argon, then AgIO₃ (848 mg, 3.0 mmol),piperidine (300 μL, 3.0 mmol) and Zn(OAc)₂ (550 mg, 3.0 mmol) wereadded. The resulting mixture was heated at 80° C. for 2.5 h. Thereaction was monitored by TLC [silica, hexanes/CH₂Cl₂, (1:1); showing asingle green spot)] and absorption spectroscopy (bands at ˜410 nm and˜610 nm). The color change of the reaction mixture from red to greenindicates the formation of chlorin. The reaction mixture was cooled toroom temperature then passed through a short column (silica, CH₂Cl₂).The major fraction was concentrated and again chromatographed [silica,hexanes/CH₂Cl₂ (2:1 then 1:1)]. The greenish blue solid obtained wasdissolved in a minimum of CH₂Cl₂ and precipitated by adding hexanes,affording a greenish blue solid (25 mg, 18%). ¹H NMR δ 2.01 (s, 6H),2.67 (s, 3H), 4.48 (s, 2H), 7.50 (d, J=7.2 Hz, 2H), 7.91 (d, J=7.2 Hz,2H), 7.95 (d, J=8.1 Hz, 2H), 8.09 (d, J=8.1 Hz, 2H), 8.51 (d, J=4.2 Hz,1H), 8.67 (m, 5H), 8.78 (d, J.=4.2 Hz, 1H), 9.56 (s, 1H); LD-MS obsd693.78; FAB-MS obsd 694.0580, calcd 694.0572 (C₃₅H₂₇IN₄Zn); λ_(abs)(toluene)/nm 411 (log ε=5.33, fwhm=18 nm), 616 (4.76), λ_(em) 619, 674nm.

Notes about chlorin formation: (1) The complete reduction of thecarbonyl in the EH precursor to the corresponding carbinol sometimesrequires additional NaBH₄. The reduction must be complete prior toperforming the chlorin forming reaction. (2) Upon workup of the EH theorganic layers were dried in K₂CO₃ (the carbinol decomposes quickly upondrying over Na₂SO₄ or MgSO₄). It is important to not take the EHsolution to dryness, as the EH in dried form is quite labile. (3) The EHupon workup, and the condensation solution giving the dihydrobilene-α,generally were either yellow or light red; these solutions led tochlorins in good yield. In some instances, further darkening wasobserved, in which case low yields of chlorins were obtained.

General conditions for demetalation.17,18-Dihydro-18,18-dimethyl-5-(4-methylphenyl)-12-(4-iodophenyl)porphyrin(22). To a solution of Zn-22 (10 mg, 14.4 μmol) in anhydrous CH₂Cl₂ (5mL) was added TFA (58 μL, 0.75 mmol). After stirring for 30 min at roomtemperature (monitoring by TLC and UV-Visible spectroscopy), thereaction was quenched with 10% aq NaHCO₃ (20 mL) and extracted withCH₂Cl₂. The combined organic layers were washed with water, dried(Na₂SO₄), and concentrated. Further purification (if necessary) wasachieved by chromatography on a short column [silica, hexanes/CH₂Cl₂(1:1 then 1:2)] affording a green solid (8.0 mg, 88%). ¹H NMR δ −2.15(br, 1H), −1.85 (br, 1H), 2.07 (s, 6H), 2.69 (s, 3H), 4.64 (s, 2H), 7.54(d, J=7.5 Hz, 2H), 8.04 (m, 4H), 8.16 (d, J=8.1 Hz, 2H), 8.64 (d, J=4.5Hz, 1H), 8.85 (AB quartet, J=4.5 Hz, 2H), 8.90 (m, 3H), 8.99 (s, 1H),9.84 (s, 1H); LD-MS obsd 633.88; FAB-MS obsd 632.1434, calcd 632.1437(C₃₅H₂₉IN₄); λ_(abs) (toluene)/nm 414 (log ε=5.13, fwhm=34 nm), 505(4.12), 643 (4.65); λ_(em) 646, 682 nm.

Zn(II)-17,18-Dihydro-18,18-dimethyl-5-(4-methylphenyl)-12-{4-[2-(trimethylsilyl)ethynyl]phenyl}porphyrin(Zn-23). Following the general procedure for chlorin formation, thereaction of 13-OH [prepared from 13 (130 mg, 0.25 mmol)] and 1 (57 mg,0.30 mmol) yielded a blue solid (36 mg, 22%). ¹H NMR δ 0.35 (s, 9H),2.03 (s, 6H), 2.67 (s, 3H), 4.54 (s, 2H), 7.50 (d, J=8.1 Hz, 2H), 7.86(d, J=8.1 Hz, 2H), 7.96 (d, J=7.5 Hz, 2H), 8.16 (d, J=8.1 Hz, 2H), 8.53(d, J=4.5 Hz, 1H), 8.60 (s, 1H), 8.68 (m, 2H), 8.73 (d, J=4.5 Hz, 1H),8.75 (s, 1H), 8.80 (d, J=4.5 Hz, 1H), 9.63 (s, 1H); LD-MS obsd 665.74;FAB-MS obsd 664.2007, calcd 664.2001; (C₄₀H₃₆IN₄SiZn); λ_(abs)(toluene)/nm 413 (log ε=5.31, fwhm=21 nm), 618 (4.77), λ_(em) 622, 676nm (Φ_(f)=0.1 1).

17,18-Dihydro-18,18-dimethyl-5-(4-methylphenyl)-12-{4-[2-(trimethylsilyl)ethynyl]phenyl}porphyrin(23). Following the general demetalation procedure, a sample of Zn-23(10 mg, 15 μmol) gave a green solid (8.0 mg, 89%). ¹H NMR δ −2.15 (br,1H), −1.85 (br, 1H), 0.35 (s, 9H), 2.07 (s, 6H), 2.69 (s, 3H), 4.64 (s,2H), 7.53 (d, J=7.5 Hz, 2H), 7.91 (d, J=8.1 Hz, 2H), 8.03 (d, J=8.1 Hz,2H), 8.27 (d, J=8.1 Hz, 2H), 8.64 (d, J=4.5 Hz, 1H), 8.84 (AB quartet,J=4.5 Hz, 2H), 8.89 (m, 2H), 8.93 (s, 1H), 8.99 (s, 1H), 9.86 (s, 1H);LD-MS obsd 604.31; FAB-MS obsd 602.2880, calcd 602.2866 (C₄₀H₃₈IN₄Si);λ_(abs) (toluene)/nm 415 (log ε=4.97, fwhm=36 nm), 506 (3.96), 647(4.49); λ_(em) 648, 685, 715 nm (Φ_(f)=0.25).

Zn(II)-17,18-Dihydro-18,18-dimethyl-5-(4-methylphenyl)-8-(4-iodophenyl)porphyrin(Zn-24). Following the general procedure for chlorin formation, thereaction of 17-OH [prepared from 17 (110 mg, 0.20 mmol)] and 1(45 mg,0.24 mmol) yielded a blue solid (30 mg, 24%). ¹H NMR δ 2.03 (s, 6H),2.67 (s, 3H), 4.51 (s, 2H), 7.50 (d, J=8.1 Hz, 2H), 7.86 (d, J=8.1 Hz,2H), 7.97 (d, J=8.1 Hz, 2H), 8.02 (d, J=8.1 Hz, 2H), 8.54 (s, 1H), 8.60(s, 1H), 8.69 (m, 4H), 8.97 (d, J=4.2 Hz, 1H), 9.61 (s, 1H); LD-MS obsd696.39; FAB-MS obsd 694.0607, calcd 694.0572 (C₃₅H₂₇IN₄Zn); λ_(abs)(toluene)/nm 416 (log ε=5.13, fwhm=18 nm), 607 (4.49); λ_(em) 609, 661nm.

17,18-Dihydro-18,18-dimethyl-5-(4-methylphenyl)-8-(4-iodophenyl)porphyrin(24). Following the general demetalation procedure, a sample of Zn-24(10 mg, 14.4 μmol) gave a green solid (7.5 mg, 83%). ¹H NMR δ −2.20 (br,1H), −1.96 (br, 1H), 2.07 (s, 6H), 2.68 (s, 3H), 4.63 (s, 2H), 7.53 (d,J=8.1 Hz, 2H), 7.86 (d, J=8.7 Hz, 2H), 8.03 (m, 4H), 8.64 (s, 1H), 8.85(m, 3H), 8.91 (s, 1H), 8.99 (s, 1H), 9.17 (d, J=4.5 Hz, 1H), 9.83 (s,1H); LD-MS obsd 631.58; FAB-MS obsd 632.1454, calcd 632.1437(C₃₅H₂₉IN₄); λ_(abs) (toluene)/nm 410 (log ε=5.11, fwhm=32 nm), 504(4.01), 638 (4.48); λ_(em) 639, 679, 702 nm.

Zn(I)-17,18-Dihydro-18,18-dimethyl-2-(4-iodophenyl)-5-(4-methylphenyl)-12-{4-[2-(trimethylsilyl)ethynyl]phenyl}porphyrin(Zn-25). Following the general procedure for chlorin formation, thereaction of 13-OH [prepared from 13 (103 mg, 0.20 mmol)] and 21 (86 mg,0.22 mmol) yielded a blue solid (42 mg, 24%). ¹H NMR δ 0.36 (s, 9H),1.96 (s, 6H), 2.67 (s, 3H), 4.48 (s, 1H), 7.50 (d, J=7.5 Hz, 2H), 7.82(d, J=8.7 Hz, 2H), 7.86 (d, J=8.1 Hz, 2H), 7.97 (d, J=8.1 Hz, 2H), 8.02(d, J=8.1 Hz, 2H), 8.13 (d, J=7.8 Hz, 2H), 8.51 (d, J=4.2 Hz, 1H), 8.63(s, 1H), 8.67 (s, 1H), 8.70 (s, 2H), 8.78 (d, J=4.2 Hz, 1H), 9.58 (s,1H); LD-MS 866.34; FAB-MS obsd 866.1257, calcd 866.1280 (C₄₆H₃₉IN₄SiZn);λ_(abs) (toluene)/nm 417 (log ε=5.32, fwhm=21 nm), 629 (4.90); λ_(em)635, 691 nm.

17,18-Dihydro-18,18-dimethyl-2-(4-iodophenyl)-5-(4-methylphenyl)-12-{4-[2-(trimethylsilyl)ethynyl]phenyl}porphyrin(25). Following the general demetalation procedure, a sample of Zn-25(11.0 mg, 13.7 μmol) gave a green solid (8.0 mg, 78%). ¹H NMR δ −1.95(br, 1H), −1.70 (br, 1H), 0.36 (s, 9H), 2.0 (s, 6H), 2.68 (s, 3H), 4.60(s, 2H), 7.53 (d, J=8.1 Hz, 2H), 7.91 (d, J=8.1 Hz, 2H), 8.03 (d, J=8.1Hz, 2H), 8.07 (d, J=8.1 Hz, 2H), 8.26 (d, J=8.1 Hz, 2H), 8.62 (d, J=4.2Hz, 1H), 8.81 (s, 1H), 8.88 (d, J=4.2 Hz, 1H), 8.91 (s, 1H), 8.95 (s,1H), 8.96 (s, 1H), 9.84 (s, 1H); LD-MS 804.02; FAB-MS obsd 804.2157,calcd 804.2145 (C₄₆H₄₁IN₄Si); λ_(abs) (toluene)/nm 422 (log ε=5.09,fwhm=34 nm), 509 (4.08), 655 (4.68); λ_(em) 660, 726 nm.

TABLE 1 Absorption spectral properties of chlorins.^(a) Soret/Qintensity Chlorins λ_(max) (nm), Soret λ_(max) (nm), Q ratio 24 409 6374.3 22 414 643 3.0 23 416 645 3.1 25 422 655 2.5 Pheophytin a^(b) 408667 2.1 Pheophytin b^(b) 434 655 5.1 Zn-24 415 606 4.3 Zn-22 411 615 3.6Zn-23 412 617 3.5 Zn-25 417 628 2.6 Chlorophyll a^(b) 430 662 1.3Chlorophyll b^(b) 455 644 2.8 ^(a)In toluene at room temperature. ^(b)Indiethyl ether (Smith, J. H. C.; Benitez, A. In Modern Methods of PlantAnalysis, Paech, K.; Tracey, M. V., Eds.; Springer-Verlag: Berlin 1955,Vol. IV, pp. 142-196).

An alternative approach to chlorins that bear a geminal dimethyl lock,avoid flanking meso and β substituents, and can be used in model systemshas involved reaction of a tripyrrole complex with a pyrrolefunctionalized for subsequent elaborations (Montforts, F.-P.; Kutzki, O.Angew. Chem. Int. Ed. 2000, 39, 599-601; Abel, and Montforts, F.-P.Tetrahedron Lett. 1997, 38, 1745-1748: Schmidt, W.; Montforts, F.-P.Synlett 1997, 903-904).

The foregoing is illustrative of the present invention, and is not to beconstrued as limiting thereof. The invention is defined by the followingclaims, with equivalents of the claims to be included therein.

We claim:
 1. A solar cell, comprising: (a) a first substrate comprising a first electrode; (b) a second substrate comprising a second electrode, with said first and second substrate being positioned to form a space therebetween, and with at least one of (i) said first substrate and said first electrode and (ii) said second substrate and said second electrode being transparent; (c) a layer of light harvesting rods electrically coupled to said first electrode, each of said light harvesting rods comprising a non-discotic backbone polymer of Formula I: X¹X^(m+1))_(m)  (I)  wherein: m is at least 2; X¹ is a charge separation group having an excited-state of energy equal to or lower than that of X²; X² through X^(m+1) are chromophores; and X¹ is electrically coupled to said first electrode; said solar cell further comprising (d) an electrolyte in said space between said first and second substrates.
 2. A solar cell according to claim 1, wherein X¹ through X^(m +1) comprise porphyrinic macrocycles.
 3. A solar cell according to claim 1, further comprising a mobile charge carrier in said electrolyte.
 4. A solar cell according to claim 1, wherein said electrolyte comprises an aqueous electrolyte.
 5. A solar cell according to claim 1, wherein said electrolyte comprises a non-aqueous electrolyte.
 6. A solar cell according to claim 1, wherein said electrolyte comprises a polymer electrolyte.
 7. A solar cell according to claim 1, wherein said electrolyte comprises a solid.
 8. A solar cell according to claim 1, wherein said solar cell is devoid of liquid in said space between said first and second substrates.
 9. A solar cell according to claim 1, wherein said charge separation group X¹ comprises a double-decker sandwich coordination compound.
 10. A solar cell according to claim 1, wherein said light harvesting rods are oriented substantially perpendicularly to said second electrode.
 11. A solar cell according to claim 1, wherein said light harvesting rods are linear.
 12. A solar cell according to claim 1, wherein said light harvesting rods are not greater than 500 nanometers in length.
 13. An electrical device, comprising: (a) a solar cell according to claim 1; and (b) a circuit electrically coupled to said solar cell.
 14. An electrical device according to claim 13, wherein said circuit comprises a resistive load.
 15. A solar cell, comprising: (a) a first substrate comprising a first electrode; (b) a second substrate comprising a second electrode, with said first and second substrate being positioned to form a space therebetween, and with at least one of (i) said first substrate and said first electrode and (ii) said second substrate and said second electrode being transparent; (c) a layer of light harvesting rods electrically coupled to said first electrode, each of said light harvesting rods comprising a non-discotic backbone polymer of Formula I: X¹X^(m+1))_(m)  (I)  wherein: m is at least 2; X¹ is a charge separation group having an excited-state of energy equal to or lower than that of X²; X² through X^(m+1) are chromophores; X¹ is electrically coupled to said first electrode; and X¹ through X^(m+1) are selected so that, upon injection of either an electron or hole from X¹ into said first electrode, the corresponding hole or electron from X¹ is transferred to at least X²; said solar cell further comprising (d) an electrolyte in said space between said first and second substrates; and (e) a mobile charge carrier in said electrolyte.
 16. A solar cell according to claim 15, wherein X¹ through X^(m+1) are selected so that, upon injection of an electron from X¹ into said first electrode, the corresponding hole from X¹ is transferred to at least X².
 17. A solar cell according to claim 15, wherein X^(m+1) is bonded to said second electrode.
 18. A solar cell according to claim 15, wherein X¹ through X^(m+1) comprise porphyrinic macrocycles.
 19. A solar cell according to claim 15, wherein said electrolyte comprises an aqueous electrolyte.
 20. A solar cell according to claim 15, wherein said electrolyte comprises a non-aqueous electrolyte.
 21. A solar cell according to claim 15, wherein said electrolyte comprises a polymer electrolyte.
 22. A solar cell according to claim 15, wherein said electrolyte comprises a solid.
 23. A solar cell according to claim 15, wherein said solar cell is devoid of liquid in said space between said first and second substrates.
 24. A solar cell according to claim 15, wherein said charge separation group X¹ is a double-decker sandwich coordination compound.
 25. A solar cell according to claim 15, wherein said light harvesting rods are oriented substantially perpendicularly to said second electrode.
 26. A solar cell according to claim 15, wherein said light harvesting rods are linear.
 27. A solar cell according to claim 15, wherein said light harvesting rods are not greater than 500 nanometers in length.
 28. An electrical device, comprising: (a) a solar cell according to claim 15; and (b) a circuit electrically coupled to said solar cell.
 29. An electrical device according to claim 28, wherein said circuit comprises resistive load.
 30. A solar cell, comprising: (a) a first substrate comprising a first electrode; (b) a second substrate comprising a second electrode, with said first and second substrate being positioned to form a space therebetween, and with at least one of (i) said first substrate and said first electrode and (ii) said second substrate and said second electrode being transparent; (c) a layer of light harvesting rods electrically coupled to said first electrode, each of said light harvesting rods comprising a non-discotic backbone polymer of Formula I: X¹X^(m+1))_(m)  (I)  wherein: m is at least 2; X¹ is a charge separation group having an excited-state of energy equal to or lower than that of X²; X² through X^(m+1) are chromophores; X¹ is electrically coupled to said first electrode; and X^(m+1) is electrically coupled to said second electrode; said solar cell further comprising (d) an electrolyte in said space between said first and second substrates.
 31. A solar cell according to claim 30, wherein X¹ through X^(m+1) comprise porphyrinic macrocycles.
 32. A solar cell according to claim 30, wherein X¹ through X^(m+1) are selected so that, upon injection of an electron from X¹ into said first electrode, the corresponding hole from X¹ is transferred to X^(m+1).
 33. A solar cell according to claim 30, wherein said electrolyte comprises an aqueous electrolyte.
 34. A solar cell according to claim 30, wherein said electrolyte comprises a non-aqueous electrolyte.
 35. A solar cell according to claim 30, wherein said electrolyte comprises a polymer electrolyte.
 36. A solar cell according to claim 30, wherein said electrolyte comprises a solid.
 37. A solar cell according to claim 30, wherein said solar cell is devoid of liquid in said space between said first and second substrates.
 38. A solar cell according to claim 30, wherein said charge separation group X¹ is a double-decker sandwich coordination compound.
 39. A solar cell according to claim 30, wherein said light harvesting rods are oriented substantially perpendicularly to said second electrode.
 40. A solar cell according to claim 30, wherein said light harvesting rods are linear.
 41. A solar cell according to claim 30, wherein said light harvesting rods are not greater than 500 nanometers in length.
 42. An electrical device, comprising: (a) a solar cell according to claim 30; and (b) a circuit electrically coupled to said solar cell.
 43. An electrical device according to claim 42, wherein said circuit comprises a resistive load.
 44. A solar cell according to claim 1, 15 or 30, wherein X¹ through X^(m+1) each independently comprise a porphyrinic macrocycle selected from the group consisting of Formula X, Formula XI, Formula XII, Formula XIII, Formula XIV, Formula XV, Formula XVI, and Formula XVII:

wherein: M is selected from the group consisting of Zn, Mg, Pd, Sn and Al, or M is absent; K³, K², K³, K⁴, K⁵, K⁶, K⁷, and K⁸ are independently selected from the group consisting of N, O, S, Se, Te, and CH; S¹, S², S³, S⁴ S⁵, S⁶, S⁷, S⁸, S⁹, S¹⁰, S¹¹, S¹², S¹³, S¹⁴, S¹⁵ and S¹⁶ are each independently selected from the group consisting of H, aryl, phenyl, cycloalkyl, alkyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl; and wherein each pair of S¹ and S², S³ and S⁴, S⁵ and S⁶, and S⁷ and S⁸, optionally independently form an annulated arene, which annulated arene is optionally unsubstituted or substituted one or more times with a substituent selected from the group consisting of H, aryl, phenyl, cycloalkyl, alkyl, halogen, alkoxy, alkylthio, perfluoroalkyl, perfluoroaryl, pyridyl, cyano, thiocyanato, nitro, amino, alkylamino, acyl, sulfoxyl, sulfonyl, imido, amido, and carbamoyl; and wherein S¹ through S¹⁶ optionally comprise a linking group covalently linked to an adjacent porphyrinic macrocycle of X¹ through X^(m+1) or a linking group covalently linked to said first electrode.
 45. A solar cell according to claim 1, 15 or 30, wherein said light-harvesting rods are intrinsic rectifiers of excited-state energy.
 46. A solar cell according to claim 1, 15, or 30, wherein said light-harvesting rods are intrinsic rectifers of holes. 